With your product upgrade the Openreach DLM may have been reset. So don't mess for the rest of the day it was changed, and a full 24 hours after. So on the third day it should be in normal running mode. (You may even be at that stage as you didn't give dates

).
See how the stats look at that time and if still top speed then I would just take the router to the extension and check how things look.
What kit are you using? If you can't see the sync, then a
BT Wholesale Performance Test followed by its
Further Diagnostics at the bottom of the page will give us the IP Profile. We can calculate the sync within a few bps from that.
The sync from the modem would be better of course.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 56203/14254Kbps @ 600m. BQMs -
IPv4 &
IPv6