Problem with old iphone

Hello everyone, so I have this old iphone 6s plus and i’m trying to use it as my phone for work but here’s the problem

Messages and cellular data work fine, but i can’t receive calls and when i try to make one, the text GPRS shows up instead of 4G and i can’t use data or messages anymore, i’m forced to turn on and off airplane mode

I’ve contacted Apple service and they made a remote diagnostic of the phone saying there’s no hardware issues, even related to the sim card reader

Then i contacted my provider and they said there’s no issues with the sim, so what i’m supposed to do?

Things i’ve tried:

Hard resetting phone

Turn on and off airplane mode

Eject and insert the sim

Restored connection settings

Restored the iphone completely, reinstalling the latest version of ios with itunes

none of them work, has anyone experienced this problem before?

thanks in advance if anyone could help me!