Not an answer to anyone’s specific problem but this was my experience.
I had a home hub 4 and modem and had no trouble until this year when the speed was all over the place, had a chat and was sent home hub 6 which made no difference.
I did the silent line test, which seemed to be ok but might have been the luck of the draw as it could have been an intermittent fault.
Had two engineer visits who got the speed up but it soon dropped again. I also started to get numerous drop outs with the broadband turning off and on.
Third engineer came and said he knew what the problem was before he entered house and that the other engineers should have sorted out the problem as they have been told to do what he did.
He changed the old (probably 50 years) flat grey cable from the pole to the house for the new black round cable.
Since then no problems and the speed, although up and down with all this home working, has increased about 10%.