I've a Vigor 2865 - primarily using full fibre via WAN2 ethernet, but with a SoGEA back-up on WAN1 (which was the primary connection for many years).
WAN1 has been absolutely stable for years until a few months ago.
Then I started noticing frequent (1 - 2 times per day) reconnects.
This deteriorated to becoming complete connection drops.
Going into DSL status shows it in a loop of TRAINING > FAIL > READY
Doing adsl reboot command on the console brings it back to life, as does disconnecting and reconnecting the DSL lead.
ISP has tested the line and says it's all good. I've tried a new DSL lead - no change, and had a new Master Socket installed (nothing else connected to it, and for a while I had the faceplate removed and the DSL lead straight into the "test" socket.
Firmware is 4.4.6.1_BT, DSL version 8D1B17_A/B/C HW: A
I'd be minded to believe it's a line fault - albeit the ISP says not, and not sure why an adsl reboot would resolve that?