I was thinking it could be a radio band issue?

I had a fun situation when I had friends visiting me in Japan for a road trip. One friend's US-model Android phone didn't support the specific low bands used for sparse coverage in rural Japan, but the repeaters inside of the tunnels were all on standard 2100 MHz, so whenever we drove through a tunnel he rushed to his phone to get some messages through. Kind of the opposite to what you usually experience with loss of signal in tunnels :)