Unlike FM, DAB radios need a good groundplane (flat metal area like a car roof) to work well. It is difficult to find such area on a bike let alone a helmet. This is undoubtedly why BMW radios (made by Alpine) do not offer DAB functionality on their tourers.
There are many stories of folks who have fitted after-market car DAB units without a properly grounded aerials and have experienced patchy reception.
I have had success by using a proper DAB car aerial mounted on a top-box and the secret was to remove the rubber passenger backrest and hide several layers of folded aluminium kitchen foil underneath. This seemed to be sufficient to mimic a metal car roof.
If all this seems rather a faff, then others have been satisfied with pocket DAB radios from the likes of PURE connected via 3.5mm jack. Maybe PURE do something with Bluetooth nowadays.....