DAB signal in the UK is very flakey at times.
Does it always happen in the same road/place? There is a road l use, and I know exactly where the signal will drop out. Very annoying.
You can enable your MMI to drop to FM if the DAB is lost - I can’t remembeer where in the menu it is, but have a look under the radio settings. It’s called auto follow, or auto frequency - something like that.
Personally I think the DAB sensitivity on the Audi’s is a bit rubbish, and there is no way to adjust (that I’ve found). I can have no DAB signal on MMI, and my portable radio, inside the car is fine