Finding the reason for not having mobile connection

I have Nexus 5 D821 which fell on the floor and lost its ability to even find a mobile network. I am now looking for tips to find out where the problem is.

So far I've verified with a (very basic) multimeter that both of the signal cables are intact and properly in place. I've also checked that the spring connectors have contact and that antennas at the bottom of the phone are not broken. The phone finds the SIM card and asks for pin code.

For me it appears that everything down from the motherboard works correctly, but of course, I can't be 100% about that. It might be that there's something wrong with the small circuit board, although I suspect it's the main board.

The phone seems to work otherwise nicely. Bluetooth, GPS, WiFi, wireless charging etc. are working as intended.

I would like to know if there's some way (software or measurement) to diagnose if there are problems with any of the chips, and what kind of measurement should I make to find out if the motherboard is broken or not, and where.

I would also appreciate if anybody had huge resolution pictures or schematics of the circuit boards so that I can verify all components are in place.