i had my screen fixed Sunday for my samsung s9+ but now my cellular network dosnt work in apartment only outside i cant receive any calls do i have to come back or i have to contact my service provider, phone was good before i had it fixed.

If the store is closer i would start there. Going thru phone provider is going to take longer and cost you shipping.

Do both

I would also do both. I'm leaning toward the repair that was done is what knocked it out

Do both go back and while there contact your provider. They might have to replace you sim or not. Din you restart your device?