Fibre speeds keep dropping, now at less than 2Mbps

Hi my fibre speedshave crashed dramatically and I am looking for some help in getting them back to where they should be.

They used to be stuck at around 18Mbps down and 1.8Mbps up. Whilst the upload speeds aren't great I'm pretty sure they have always been around 1.8-2Mbps.

I should be getting between 30-40Mbps download speeds and this what I got when I first signed up, it was great for a few months and then it become stuck at 18Mbps for month for some unknown reason.

I left it for a few months longer to see if it would sort itself out but the speeds never improved. So I finally decided to turn the router off and leave it for about 30 mins before turing it back on. The speeds went back up to 30+Mbps which was great but then after a while it started to crash again and now its at 1-2Mbps.

The connection has always been stable and never seems to disconnect. The router has only been disconnected when I have manually restarted it a couple of times to try and fix it. I can count the times I have restarted the router on one hand as I know DLM might think there's a problem otherwise.

It seems restarting the router or changing Wifi channels restores the speed back to 30+Mbps, then about 10mins to a couple of hours later it goes into freefall and the speed gets trashed.

Phone line is fine and there is noise on the line.

This seems to only effect the download speed, upload speed has always been consistent.

Tried multiple devices.

Tried turning the router off for 30mins-1hour.

Tried various locations in the house and testing and different times of the day.

Tried wired and wireless connection

Connected to master socket.

Tried using different filters, those supplied by talktalk and our previous ones from BT.

I have run multiple speed tests using talktalks broadband speed checker so you can check the results of those.

Fibre medium package.