Data and service not working

My phone started having sporadic issues with data and network, wifi always works. However, it now seems to be a consistent issue. I feared it was the SIM so I changed providers and still have the same issues.

The back of my phone is cracked and apparently fell off awhile ago but wasnt an issue because I have a phone case. However, now I am experiencing issues and fear its an antenna issue.

How do I know what the cause is and how do I go about fixing it? I can fix it myself probably, I am not afraid of a DIY. but my phone is unlocked and I do not have insurance on it so my provider wont fix it. I just want to know what the issue is so I can try to fix it