iPhone 5c can't find service, where do I find the GSM Antenna?

Hey guys, I got a defect iPhone from a friend that I want to repair. The activations fails because it needs a sim card for that. It doesn't matter if I put it in or not, it's always searching for service.

The Wifi is fine. The sim card works with other devices. I tried calling an emergency number for testing, the call instantly fails. That's why I think the antenna is broken. I don't think that it has a problem with reading the sim card as emergency calls should work without one.

I went looking for a GSM antenna, but I was only able to find Wifi antenna parts. I hope you guys can tell me which part I need and if the problem might be something different (software->new motherboard maybe?)

Thank you in advance!