iPhone 7 No modem firmware

Hi, my Iphone 7 suddenly started showing “no service” and no number after modem firmware, I went to an affiliated apple store and they told me that it is a common problem where apple has set up a free repair program.

However, my phone was not accepted because there is a small crack on the screen, the size of a grain of rice (which has been there since way before the problem showed up). Their excuse was that it could be “my fault” and is not legible for the repair program.

Of course I bought a new phone, but I still do want to repair this one (for: fun, experience and e-waste reasons). There are some video’s on YT of people doing this, but I would like to follow a manual of some kind.

Is there a place where I can find such manual? Or could someone tell me: what to look for, how to diagnose the problem (burnt chip/capacitor…) and how to replace this with what parts?

Many thanks in advance!

PS: The repair shop offered me a screen fix for €150, then I would be accepted in the repair program, which is ridiculous… ahahah