internet provider in Etterbeek again

Dear expats, thanks for the advise about choosing an internet provider in Etterbeek. However, at Numericable's office they explained that they were not available in Etterbeek. Since Mobistar is cheaper than Telenet, I ordered Mobistar fixed internet at home. Soon, however, I started to receive letters about some phone mobile internet subsribtion. A call to their office clarified that they did not provide fixed internet services anymore and that I have to pay a fine of 150 Euro for cancellation of the mobile internet. Any ideas how to deal with it? Meanwhile I have Telenet at home and it works well.