Contract cancellation refused same day of purchase

Today I signed a contract of a modem router for my girlfriend at around 12pm and I called my girlfriend to tell her that I've bought a Telekom modem and she told me that telkom doesn't work where she stays and I quickly went back to telkom within 30 minutes time of the purchase and they told I will have to take the modem with me and pay for it including the cancelation fee which sounded unfair to me and I left the modem with them and told them to cancel because I haven't used the modem and I want to cancel because it wont work and I will be paying for nothing for 24 months but still they called me after I left and told me I must not be surprised when I get deductions month end because I need to pay for the router. Please assist me Peter