Why doesn't the cellular work?

Everything works on the iphone except for the face ID and just now the cellular stopped working. I took it to an apple store but He suggested a whole repair of the phone. We went through a diagnosis on the store and it seemed that the antenna was faulty. I can't seem to find said antenna or anyone else with my issue. I would like to keep the phone working.

Does anyone know how to fix it? I will replace any part if needed. But I would like to know what part😅

Ps. Bluetooth and wifi work fine.

Thank you