In the master socket the cables should be connected to terminals A&B and @Keith_Beddoe suggesting only 1 wire may be connected which allows broadband to work but slowly. If you check and that is the case then just connect the loose wire to the terminal
Its more likely to be a disconnection on the external network, difficult to prove these days, as no dial tone from exchange side, one of the problems with the broadband only service.
Its below the handback speed, so if you still get the same speed at the test socket, and both wires are connected, then you would need to report it as a fault.
With no voltage on the line, it must be difficult for Openreach to find the fault.