Current Cardiff uni student here!
I'm sorry that you're in this situation. I've found flat mates last minute before through student accommodation facebook groups. Good options include Cardiff Students House Hunters or Overheard at Cardiff University. For this, I would just advise you to do your due diligence, triple-check the contract, be very aware of scams and quality of the house/ flat and try to have multiple calls with the other tenants and meet them if possible.
All the best
Jaz - Cardiff student rep