Weak Mobile connection over Sim (has to be Hardware)

Hi I have been fighting with weak Mobile Network connection for a long time and finally realized that it is my Hardware and not my Provider.

What are the possibilities of things that could be broken. I do not understand what components are in the phone to make this error and need advice.

I am asking for guidance as I only want to open my phone once and want to limit the time that I can not use it and the amount of times that I open up the phone.

And advice is appreciated

Regards Jonathan