Unauthorised debit order

I recently opened a discovery bank account because I was interested in saving and their vitality product, I called to enquire about vitality membership but wasnt happy that the amount kept increasing.I had primary account app open and that time but didnt agree to the deal,to my suprise they still managed to create a debit order with no mandate.I only saw it when it didnt go through.My crisis is that I cant get assistance on the app because my front screen is cracked and there isnt any alternative way of getting hold of them