iPhone has IMEI but no service or modem firmware

This would defiantly be a baseband issue although the Intel versions don’t usually suffer from this problem unless someone has replaced the audio IC and put too much heat on the board causing solder bridges under the baseband CPU.

ZXW has a comprehensive board view for the Intel now. I am not sure what lines you checked but I would start at the baseband PMU and then to the baseband CPU. If nothing rears its head I would replace the baseband PMU if that didn’t resolve it a reball of the baseband CPU would be the only other thing to try.