Stripe won't accept a seller's sort code when trying to on-board as a connected account on my market place

My first seller is trying to get through the onboarding process with Stripe to set up a connected account and can get payouts etc from my marketplace app.

They’re having an issue where they use SumUp which is widely used by many small businseses here in the UK but the sort code that it provides isn’t being accepted by Stripe.

Has anyone experienced this and, if so, can they suggest a way around it?

I have spoken to Stripe and they’ve said that “SumUp isn’t actually a bank…which is why it’s probably failing” but SumUp say that they do integrate seamlessly with Stripe so I don’t know where to go with this.

Any help would be much appreciated