The line didn't "retrain" for the same reason as your friend was wrong
The line sync/calibration is between the Openreach modem and the FTTC cabinet. Nothing at all to do with anything you hang on the end of the Ethernet output from the modem.
Once the line is in sync between them, the router supplies your ISP username and password and whether it is a PPPoA or PPPoE connection, and a few other parameters to do with making the connection to your ISP and beyond.
My broadband basic info/help site - www.robertos.me.uk
| Domains,website and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 53.4/16.8Mbps @ 600m. - BQM
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- Jean-Antoine Petit-Senn.
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