How can I submit a complaint?

I chose a special offer for subscription with 6 months half price and booked it. When I was filling my details I was asked a question if I would like to have a flexible cancellation of the contract and be able to cancel it with monthly notice period for additional cost of 3 EUR so I chose that option also.

Now I received my first invoice and the first promotional offer is not included but a standard price. I called to KPN and was told that because I chose the option for flexible cancellation, the initial offer doesn’t stand.

The thing is that wan’t mentioned anywhere while booking my contract and I got to the break-up option from booking the initial offer. For me this is a hiden condition, not being mentioned anywhere and you get to sign a contract, different than the stated one in the offer. 

I would like to make an official complaint, would you advice where to write?

Otherwise all is perfect with the connection.