So what do I do? Where do I get a filter from? I assume from what you are saying that the filter converts from telephone socket to an rj11 socket. Do I therefore plug the modem/router cable into this? What am I looking for? How does it test anything?
Did you not use a filter with your broadband before you moved to fibre?
as you have a separate modem and hh4 you would need to connect the rj11 cable which currently connects modem to socket to the filter which you connect to the test socket. So sequence is
socket/filter/rj11/ modem/Ethernet cable/hub