A contract is a two way agreement between the signatories(lm not a lawyer). By signing the document, you are agreeing to pay them the stated amount of money and they are agreeing to provide you with stated service.
You can prove to them that you have met your side of contract by producing receipts, and they also need to prove that they have met there side of contract failure to do so is breach of contract.
lf l were you l would take the dongle to there offices and they need to prove that it can download at the stated agreed speeds, failure to do so in there offices means you cannot do it anywhere else.
Or simply just stop paying and wait so see if they have the guts to take you to court.