Can I flash a separate radio image and have a working radio?
(Previous) Nexus 4 owner reporting in.

People at XDA had long been using hybrid modems with custom ROMs like CM. It seems specific versions are needed for each major Android version (JB/KK/LP+), but there's a nifty app right here you can try out: LINK: Google Play.

Also, there's no special radio for CM13. No custom ROM team makes radio themselves. Just flash back to a stock radio if things go wrong.