Malfunction Phenomenon
iPhone 7 No Service
But the IMEI and modern firmware information was normal in the general information list and also when dialing *#06#.
Trouble Shooting
According to the malfunction phenomenon, we can conclude that the Baseband part was normal, the problem was from the Radio Frequency (RF) circuit.
Take out the motherboard and checked the RF chips, there are two RF chips on the motherboard, XCVR1_RF, and XCVR0_RF.
The outlook looked good. The voltage for the chips is also normal
so we can try to replace XCVR1_RF and XCVR0_RF one by one.
After replacing XCVR1_RF, assemble the phone and test, the service back to normal, problem resolved.