@RedRoger 

Hi, have you tried to turn off the Mesh Technology on the Redmi 9 to have a look?

And may I know the operating system on the Redmi 9 as well?

@TP-Link 

Disabling mesh didn't help. And this also is a problem - switching off leads to situation when phone is connected to a unit with a very poor signal and WiFi speed and quality is bad.

Android version is 10 QP1A.190711.020

@RedRoger 

Thank you very much for the updated information.

 I would like to follow up on your case by email and please have a check of your email box later;

Thanks a lot.

@TP-Link 

I have a similar problem with poco m3 for 5 g. wifi system deco m4. I'm already clueless. upgrade the latest firmware deco 1.5.0 mesh roaming off. without success. if i leave the network only 5ghz and the guest network only 2.4 gHz so it works a bit better. if it can't efficiently jump between bands with the same SSID.

I have a similar problem with redmi note 10 5g. Cannot get a stable connection with Deco. As I need to use phone to control Deco, this is a major sticking point. This is a new phone. My older Moto phone g5 s plus worked fine. The phone works fine when connected directly to main router, so problem has to be with Deco. I have tried QoS and reserved address with no difference. Come on tplink, get onto this problem, as it will only get worse as more people upgrade their phones. Thank you.