The current owner of the service REFUSES to change the account name and reconnect the service.which would be the simple solution..
As I have said before, the previous occupants terminated their contract with the altnet, so there is nothing left to transfer even the parties were willing to novate the contract (which is unlikely; it is difficult to do novation in a legally-binding way, and for something like broadband it is much cleaner for the departing party to terminate and the arriving party to take out a new contract).
It appears that nobody you have contacted so far has the right to enter into new service contracts on this altnet - because, from a legal perspective, the new resident is in no different a position than anyone else seeking a new connection via that service. The existence of a potentially reusable physical connection does not change the legal position.
If, as has been suggested in the thread, new connections are not currently possible on this infrastructure, then that is the end of the matter.