IMEI Unknown

Bought Nexus 5 on Ebay a few months ago. Rooted it, installed custom ROM (Lineage 14 among others) debloated, etc, all worked fine until recently when it stopped registering that there was a SIM card in the phone. Closer examination showed complete absence of IMEI number and Baseband version. Tried various resets up to re-flashing stock ROM and unrooting but no improvement. Took it to the local mobile (that's UK'ish for cellphone) shop. When he plugged it into his laptop said the phone software was discontinued and he couldn't do anything.

Any advice?