Is it possible to fix an unlocked Xiaomi device at an authorized service center ?

My grandfather has an old redmi note 9 pro which was slowing down so much and was having a lot of issues with call connectivity.

Outgoing calls won't connect and incoming calls would hangup automatically, so It was kind of impossible to use the phone for calls.

I unlocked the device and flashed a few roms based on android 13 and android 14, but all of them had the same issues with calls.

So if it is a hardware problem, will Xiaomi service centers accept it ?