Have just read this unhappy saga.
I had a similar issue some time back when moving from copper cable to fibre.
Engineer visited and quickly found that that the issue was with the Master Socket.
The Engineer fitted an adaption to the Master Socket. It was however necessary to connect the Hub direct into the Master Socket and not into any of the extension sockets,
The Extension sockets remained active for telephone and Internet but Internet was only at around 14 mbps as previous. With Hub direct into Master Socket I then got (and still get) around 37 mbps
Well good news, eventually. Engineer arrived today but mentioned he was an apprentice, but to give him his due he went through everything from the cable in the house, the telegraph pole, the cabinets and exchange but couldn't find a fault that was causing the slow speed. While he was working his supervisor arrived and asked everything that he had done, he went through the list and said he couldn't find out why it was slow, supervisor says, "the DLM probably needs resetting" and duly dials a number on his phone and goes though a menu system pushing various numbers, within two minutes my speed had gone from the 5.199 up to 41.122mbps🎉
He said the faulty router back in July probably caused the speed drop but it needed manually resetting to get the speed back up, I only wish he had been the first one to come out all that time ago. He did warn that the speed may drop in the next few days while it finds its level, before all the problems I used to get around 24-25mbps.
So I'm happy with the speed I am getting but not at all impressed with what it took to get it back up to speed.