Hello

I have a network problem. The phone displays 4G coverage and when the call changes to 2G and can not be called. When I have a full 4G coverage it works fine.
I changed the operator to another and it is the same. Similarly, the Wifi connection works. This started to happen after removing the screw, which was loose between the antenna and the motherboard. With good coverage, the phone works without a connected antenna.
Is it an antenna or motherboard replacement / repair?
Can an admin help me?

I use google translator

This can be several reasons. Can you replace the screw to the place it belongs?