New here but ran across this thread while trying to figure out the problem on my 2018 2500HD. I had the exact same issues. Stopped to fuel up and noticed the volume wouldn’t increase and actually got quite past 40.
Racked my brain and realized the volume controls while using Apple car play and navigation via iPhone were independent of the radio volume, meaning when the navigation voice is playing the volume knob will adjust independent of the radio.
So so I plugged my phone back in hit a destination waited on the voice to come up and adjusted the volume and voila! Radio is functioning as should. I think this is probably a software issue, resetting the radio didn’t help nor did disconnecting the battery. Hopefully this helps the next person having the issue.
