I agree with the above, but you don't have to act immediately. After your contract is complete you can continue to use your TT connection on a month by month basis without renewing the contract. From your potted history it looks as though the reason for the instability was never properly investigated - changing modem/routers is the least likely solution. It's far more likely to be something in your own wiring or in the connection between the cabinet and your home, and changing ISP will make no difference to a physical defect of this kind.
So I would spend some time diagnosing the cause of the problem. This will require some commitment on your part, but there's plenty of good help available here, and the end result is likely to be more useful.