Warranty is irrelevant, no matter what the dealer says. That's simply not fit for purpose, nor of reasonable quality. Take it back to the garage, let them decide what they want to do. You should have ZERO costs for this, no matter what their warranty terms are, you're covered by the SOGA in this, not the warranty terms.
Take it back, explain the problem, see what they say. They should fix it, however they may simply just refund you and then you'll have to get a new one. That's up to them, you cannot force them to fix it, but they MUST either fix or refund. No alternative.