I can only give my own personal opinion but I have always found the Motability customer service operators unfailingly helpful. I make a point of being pleasant and to the point, and I have never failed to get a satisfactory outcome (I’ve been on the scheme for almost thirty years).
In this case I would clearly report the problem to an operator …. point out how essential RAC back up is to a disabled person …. insist on a call back when the RAC registration has been successfully sorted.
Job done, and all in a very polite and friendly manner.