If you can’t trade with the platform you BOUGHT it with, then it is no longer the SAME product you purchased. It has substantially changed to something you did NOT purchase, and you don’t have the tools and experience to desire to use a diminished product. If they refuse, call your bank and look into a charge back (but that would be a very last resort). Usually if you are a bit more stern and put it in those terms, IF they are a reputable prop firm, they should give you a refund.