When does a fibre connection contract become binding?

When you receive an offer, a bid, from a seller, it is not binding until both you and the seller explicitly accept and confirm the agreement.

If a dispute arises, the fiber provider/seller must be able to prove that a binding agreement has been entered into by proving that you have explicitly accepted the agreement. To be able to prove that a binding agreement has been entered into, you must be able to prove that the company has given an acceptance – confirmation – to you.
An agreement is binding for both parties. The provider may not change prices and terms except in exceptional cases if it is not stated in the agreement.