I upgraded from 100 to the full fibre 500 back in May. Had an engineer install new line and BT Smart hub 2. Since then I've had no end of problems with speed dropping from avg 450mbps to 50mbps both using wi fi and ethernet. A router reboot seems to solve issue but then within hours it drops again. I shouldn't have to keep rebooting router. Same again today so spent an hour speaking to bt tech guide. She rebooted router so speed wrnt back up again. Said there was WiFi congestion so she did factory reset on router and congestion cleared. Regarding ethernet, as I have an ethernet port extender in port 1 of the router she said the system doesn't like extenders so I should unplug it!! Don't believe this. She is sending me a new router smart 3 to try
We're having similar issues with our full fibre 900. Since early to mid August our speed has been erratic to say the least - 900 mbps one minute (on a wired connection), then below 1 mbps the next. Openreach have been our five times so far, sixth visit tomorrow, and insist that there is no fault, but we're on dial-up level speeds here most of the time. ONT has been replaced, cable at the grey box has been re-spliced, Smart Hub 2 has been replaced, and it's no change. BT have looked at our speed when we've been on the phone reporting the issue and they've seen that the speed going to our hub is up and down and they've said it shouldn't be doing that, so clearly something is wrong but we're getting absolutely nowhere with trying to get it fixed.
Everything is set up as it should be. Speed is equally erratic when testing it on two wired laptops, a wired desktop, two mobile phones, and a smart TV (iplayer buffering so much it's unusable, but again was fine before August). Nothing has changed in the house. Weirdly, there is one other house on my street that also has fibre (via Vodafone) and theirs started doing the same thing as ours at the same time as it happened to us, and new routers and various Openreach engineers haven't fixed it for them either. Feels like we're running round in circles and getting nowhere.
I've now seen a number of people having the same issue with BT. I'm monitoring mine for a few days, see how it is after I get a new router then if no better I will ge back onto BT.