The iPhone 7 had a flaw in its assembly where the radio IC was not epoxied to the PCB correctly. Overtime due to heat stress the chip would get fractures in its solder joints and the phone would lose the ability to get on cell networks.

I took my 7 to the Apple store in 2019 with this issue after giving it to my SO. I was told by the store rep that it was an issue with "the third party manufacturer" (Qualcomm) but Apple would do the repair for free. I learned from some independent repair communities that this was actually a really common issue and Apple was doing some silent recalls.