"Invalid Sim" even when IMEI number is correct On a recently purchased Sprint Galaxy Note 5, I installed an engineered version of Android 6.0 that apparently breaks the unlock it had before (this wasn't mentioned in the instruction so..), so after installation I get an "Invalid SIM card" notification; I checked the IMEI and it is correct, the same as the IMEI I had before, though I read that the Invalid SIM card warning comes only when the IMEI is lost because of similar installations. 
My question is, what could possibly be the problem as the IMEI is correct yet the SIM card is not recognized? Could it be that the phone's designed for Sprint and IMEI is not involved? Or could it be an easy-to-fix problem?
Partial answer:

A physically damaged SIM card can lead to the "Invalid SIM card" message as well.