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.
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If you never think of anything off the wall, you'll never think of anything original.