Roms don't overwrite you modem/radio partition. What you are referring to is usually Kernel(boot) partition related IIF its that or it could be carrier related(sim/signal, etc)I wonder what this ROM is based on modem wise. I have been testing many of the custom ROMs now that I no longer need authenticators for work and can flash freely again and this ROM has significantly worse WiFi then any other ROMs like AICP for example. I can't even hold a connection to my AC/AX WiFi at ranges other ROMs would still have 2-3 bars. It has 1 bar at best and constantly disconnects me from WiFi.
Since this is still a completely empty install with a factory reset I think I'm going to flash a different modem.bin and see if that helps.