I dropped my iPhone 12 mini

I dropped my iphone 12 mini and the back glass smashed, it was in a case and the rest of the phone is in good condition (front not smashed at all).

I picked it up and saw I had no mobile data signal, wifi or bluetooth available to use.

I took it to apple and they weren't much help as the back is quite smashed with no signal they couldn't guarantee a £250 back glass repair would even fix it. Does anyone know what could be wrong with the phone. I have a spare and am interested to repair this one myself. Especially after the service at apple.