The router might re-sync. It would not re-boot.
There is no such thing as a training period in the sense that a normal human such as you and I understand the word. It was a myth on ADSLx and is specifically not in the product specification of FTTC by Openreach. See BT SIN 498
, Section 2.2.
The 10 days on BT Wholesale circuits is and always was a time when the lowest sync in that period is recorded and becomes the Maximum Stable Rate (MSR), from which the Fault Threshold Rate (FTR) is calculated as a percentage. What that percentage is on FTTC I don't know. On ADSLx at one time it seemed to be 70%, and later 80%.
Those figures are used within BT Wholesale (and by ISPs using them) to assess the seriousness of problems reported by users.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
If you never think of anything off the wall, you'll never think of anything original.