If your old router was faulty and showing errors to BT's line management system there is a good chance that DLM (Dynamic Line Management) would have reduced your speed to try and stabilize the line because of the errors it was receiving.
If a new router has "solved" the problems it can take BT's fibre DLM a few days to recognise that the line is now fault free and increase your speed.
It might be worth waiting a couple of days and if there is no improvement then try switching the router off for 30 minutes as that can sometimes give DLM a much needed kick.
If it is still slow after that then post back so that an OCE can have a look to se if there is still any errors on the line.
Unfortunately the fibre DLM is totally controlled by BT Openreach, so TT cannot adjust this manually.