iPhones (have tried 13 and 14)  keep disconnecting from TL-MR6400. 

Hardware version: TL-MR6400 v5.3 00000003

Firmware: 1.6.0 0.9.1 v0001.0 Build 230418 Rel.34085n

What I have tried up to now:

1. Firmware is confirmed by the router to be as up to date.

2. Reboot to factory settings was done already.

Please advise on how to solve this problem which has turned very annoying.

Thanks.