@Saas1 I fear that the security deposit was not allowed (as Airbnb can't see it) and so you are in the hands of the host who appears to be taking advantage of you. Assuming you know the address of the host and their name on the bank details you are in a strong position to issue a small claims court claim against them. This is an easy process all done on line. Your best next step is to warn the host that, if they have not repaid you within a week (or2?) then you will issue proceedings at which point they will be liable for the deposit plus court costs. Hopefully this will encourage them to do the right thing and give your money back.
Airbnb are not really involved but if the host is not authorised to take a deposit (some are) then Airbnb should really be taking action against them for receiving money off-platform. If you explain to the host that as well as court action you will be ensuring Airbnb take action it may help them see sense.