Current Cardiff uni student here!
I'm sorry that you're in this situation. I've found last-minute flat mates before on student 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, watch out for scams, triple-check the contract and quality of the house/ flat and try to have multiple calls with the other tenants and meet them if possible.
I hope this helps!
Jaz - Cardiff student rep