I have identified what is causing the issue after testing with a different AP.
The EnGenius ENH200 initially had the same issue but after comparing settings with a different AP that worked, the issue is the 2.4GHz WiFi channel width. Lumus works fine with 20MHz channel width but not properly with 40MHz.
The tp-link X55/X50/X20 do not allow user to change its 2.4MHz WiFi channel width and automatically set the value based on its own algorithm. Most of the time it is set to 40MHz.
This becomes an incompatibility.
20 MHz channel width is the usual recommended value since 40MHz may not always work with everything.
So, either Lumus needs to allow the support of 40MHz channel width or tp-link support allowing users to change the channel width for its 2.4 GHz WiFi or BOTH.

Thanks.
Gan

Hello Reolink support and anyone interested,

This happaned to me yesterday (16 March 2024).

All the Reolink Lumus cameras cannot be accessed by the Reolink App.
Tried many things and eventually figured out possibly to be lost packets or filtered packets (or even the bit patterns) caused by updating the firmware of TPLink Deco X55 setup as access points. Tried turning off the fast roaming and beam forming for the X55 and even wifi Security (WPA2, WPA etc) and same issue.
From the client list in the TPLINK Deco app, all the Reolink Lumuses ARE connected.
The issue even disguised as Reolink having issue accessing the microSD cards but this is NOT the issue but it is a symptom.

What I did is to temporarily setup another access point in the middle of the house using OTHER brand APs like AIRLink, DLink or EnGenius and this has now temporarily solved all the connection issues with the Reolink Lumus cameras.
It was a big mess trying to change the WiFi setting of all the Reolink Lumus to another WiFi SSD. I had to turn off all the TPLINK Deco WiFi SSIDs and setup a temporary AP with the same SSID and then access the cameras and change their WiFi to connect to a different SSIDs. At least, I did not have to touch the cameras (change their orientation, reset and reconfigure from scratch to change the WiFi config, adjust them back to their orientation).

Another interesting data point: The Reolink Video Doorbell WiFi and the Reolink Fisheye (FE-W) DO NOT have issues at all and they both still working fine after the TPLINK Deco X55 firmware upgrade.

In Summary:
1a.) after upgrading the firmware of all the TPLINK Deco X55 units to
Firmware Version: 1.2.0 Build 20231229 Rel. 43148,
ONLY the Reolink Lumus cameras will have this issue.
All other IoT devices like locks, wall switches, smart plugs, garage door openers have no issue whatsoever.
Also, the Reolink Video Doorbell WiFi and Reolink Fisheye (FE-W) continue to work properly.

1b.) The same issue happens to the Reolink Lumus when the TPLINK Deco X20 units are upgraded to
Firmware Version 1.5.3 Build 20231227 Rwl. 62097.

2.) Change the WiFi config of the Reolink Lumus to connect to the SSID of other brand access point.

3.) Until both companies (Reolink and TPLINK) sort this out, cannot connect Reolink Lumus to the TPLINK Deco.
OR,
DO NOT upgrade your TPLINK X55 Deco to Firmware Version: 1.2.0 Build 20231229 Rel. 43148
DO NOT upgrade your TPLINK X20 Deco to Firmware Version 1.5.3 Build 20231227 Rwl. 62097
By the way, for Reolink Lumus camera System Info:
Build No. build 21052800
Hardware Ver. IPC_325C7
Config Ver. v2.0.0.0
Firmware Ver. v2.0.0.705_21052800
Detail IPC_32516MS15W1P2110000000100

The TPLINK Deco X55 update release notes:
source:  www_tp-link_com/us/support/download/deco-x55/v1.26/#Firmware

Published Date: 2024-02-26   Language:  Multi-language   File Size: 35.41 MB   
New Features/Enhancements:
 1. Added Camera Security feature.
 2. Added Wi-Fi Access Control feature to support block list and allow list.
 3. Advanced Parental Control subscription added APP block feature, which supports 2000+ applications.
 4. Added Portal authentication for Guest Network.
 5. Added detection of Ethernet backhaul link speed.
 6. Added the support for Matter protocol.
 7. Add the module of Eco Mode, Wi-Fi Schedule and Auto Update.
 8. Optimize the collaboration with Tapo.
Bug Fixes:
 1. Fix some bugs related to roaming when working with some Intel network cards.
 2. Fix the potential security vulnerability and its related problem.
 3. Fix some bugs of potential problems related to algorithm.
Note:
 This firmware can not be degraded to the previous version.

Thanks.
Gan

@gan_719632026226894
By the way, for Reolink Lumus camera System Info:
Build No. build 21052800
Hardware Ver. IPC_325C7
Config Ver. v2.0.0.0
Firmware Ver. v2.0.0.705_21052800
Detail IPC_32516MS15W1P2110000000100

The TPLINK Deco X55 update release notes:
source: www_tp-link_com/us/support/download/deco-x55/v1.26/#Firmware

Published Date: 2024-02-26 Language: Multi-language File Size: 35.41 MB
New Features/Enhancements:
1. Added Camera Security feature.
2. Added Wi-Fi Access Control feature to support block list and allow list.
3. Advanced Parental Control subscription added APP block feature, which supports 2000+ applications.
4. Added Portal authentication for Guest Network.
5. Added detection of Ethernet backhaul link speed.
6. Added the support for Matter protocol.
7. Add the module of Eco Mode, Wi-Fi Schedule and Auto Update.
8. Optimize the collaboration with Tapo.
Bug Fixes:
1. Fix some bugs related to roaming when working with some Intel network cards.
2. Fix the potential security vulnerability and its related problem.
3. Fix some bugs of potential problems related to algorithm.
Note:
This firmware can not be degraded to the previous version.

Hello Reolink support and everyone interested,

This happaned to me yesterday 16 March 2024).

All the Reolink Lumus camertas cannot be accessed by the Reolink App.
Tried many things and eventually figured out possibly to be lost packets or filtered packets (or even the bit patterns) caused by updating the firmware of TPLink Deco X55 setup as access points. Tried turning off the fast roaming and beam forming for the X55 and even wifi Security (WPA2, WPA etc) and same issue.
From the client list in the TPLINK Deco app, all the Reolink Lumuses ARE connected.
The issue even disguised as Reolink having issue accessing the microSD cards but this is NOT the issue but it is a symptom.

What I did is to temporarily setup another access point in the middle of the house using OTHER brand APs like AIRLink, DLink or EnGenius and this has now temporarily solved all the connection issues with the Reolink Lumus cameras.
It was a big mess trying to change the WiFi setting of all the Reolink Lumus to another WiFi SSD. I had to turn off all the TPLINK Deco WiFi SSIDs and setup a temporary AP with the same SSID and then access the cameras and change their WiFi to connect to a different SSIDs. At least, I did not have to touch the cameras (change their orientation, reset and reconfigure from scratch to change the WiFi config, adjust them back to their orientation).

Another interesting data point: The Reolink Video Doorbell WiFi and the Reolink Fisheye (FE-W) DO NOT have issues at all and they both still working fine after the TPLINK Deco X55 firmware upgrade.

In Summary:
1a.) after upgrading the firmware of all the TPLINK Deco X55 units to
Firmware Version: 1.2.0 Build 20231229 Rel. 43148,
ONLY the Reolink Lumus cameras will have this issue.
All other IoT devices like locks, wall switches, smart plugs, garage door openers have no issue whatsoever.
Also, the Reolink Video Doorbell WiFi and Reolink Fisheye (FE-W) continue to work properly.

1b.) The same issue happens to the Reolink Lumus when the TPLINK Deco X20 units are upgraded to
Firmware Version 1.5.3 Build 20231227 Rwl. 62097.

2.) Change the WiFi config of the Reolink Lumus to connect to the SSID of other brand access point.

3.) Until both companies (Reolink and TPLINK) sort this out, cannot connect Reolink Lumus to the TPLINK Deco.
OR,
DO NOT upgrade your TPLINK X55 Deco to Firmware Version: 1.2.0 Build 20231229 Rel. 43148
DO NOT upgrade your TPLINK X20 Deco to Firmware Version 1.5.3 Build 20231227 Rwl. 62097

Thanks.
Gan