The dongle problem has been resolved by you buying another. If your current dongle develops a fault, you need to resolve that with whoever you got it from. If you got it from Three and if it was within a couple of months of getting it, personally i would give Three the choice of either replacing the dongle, repairing it or ending the contract.
The coverage maps are not reliable (particularly for Three) so you might find that there is coverage at the new address. If not, in my opinion, you should either complete the contract or pay the "buy out charge".
I doubt that they will put the suggestion "a friend or a family member can take over the contract" in writing but, if they did, you could simply tell them that your Uncle Bruce in Australia was taking over the contract and then cancel your payments.