Why does my phone say no service

Cell phone towers don't provide 100% coverage, especially if you are indoors. You can have your settings correct, and your plan all paid up, and still encounter "dead" areas.

I see that the iPhone 7 supports a mixture of 3G (GSM, CDMA) and 4G (LTE) cellular bands and frequencies. The phone companies have shut down their 3G networks, so your iPhone 7 is relying on its 4G support – which might not be as extensive as the 4G and 5G support in newer iPhones.

If your iPhone says "No service" all of the time, that's a matter I would suggest taking up with the phone company.

iPhone 7 - Technical Specifications

Start here→If you see No Service, Searching, or SOS on your iPhone or iPad. If following that guidance doesn’t resolve the problem, post back with how far you got and what happened.

See if this applies to you -

iPhone 7 Repair Program for "No Service" Issues - Apple Support