Internet won't be possible at my house until May, but mobile subscription is active and more expensice than expected

On the 14th I went to a KPN shop to do a Wifi contract for my house. Firstly, the employee didn't even check if my house was suitable to receive the modem and have internet (which it turned out it wasn't, the third time I went to the shop to ask for support).
On the 14th I ended up buying a new phone and also making a SIM card with KPN, because the employee said that as I was going to make an internet contract I would have a discount for my phone and pay 24€ (a discount for having wifi at home with KPN too). I thought it was great and ended up signing a contract too. To this day I haven't received the modem to have internet at home, I went to the shop to find out why and was told by another employee that my house isn't ready for internet and it won't be possible until MAY. To add to all this deception, I received an invoice to pay for the telephone now and to my amazement I was charged 32.50€ instead of 24€. I'm asking for a refund because the contract I signed was for €24 and by mistake and misinformation on the part of the salesperson, I was led into a huge error.