I haven't moved between jobs to suit me though, I had no choice when I returned due to the risk assessment, I didn't mind going straight back to the customer facing role.
I'm not criticising - simply stating a fact - and you have. You were recruited to the role that you now don't want. You asked for a move - that move suited you but the employer let you move when they didn't have to. Equally, although it would have made very little sense in some ways, the employer was at liberty to say sorry, but it isn't our fault you are shielding / more vulnerable, we have no role that meets the risk assessment so sorry, you are dismissed. They didn't. Then you wanted to go back to the customer facing role, and again they let you. I totally get why, and I am not saying you did anything wrong, but being dispassionate about this, your role -the one you were recruited to - is not the role you want, and for various reasons that were about flexibility for you (not the employer) they have let you switch backwards and forwards. They didn't have to do any of that.
Now they are saying that for the needs of the business they need you in the original role. Now you can try to find a compromise, or you can accept that this is what it is (at least for now) or you can find another job (in which case they will simply then have to replace you).
You may not like it, but as someone else said, they haven't done anything wrong, and all things considered they have been somewhat flexible and fair when they didn't have to be. But they need this role doing, and for now you are it.