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I've been having this issue with my router rebooting several times a day.
Thinking it might be the VDSL part malfunctioning, I got an HG612 and hooked it up to the vodafone router's WAN port. This seemed to work fine connection wise; however, rebooting still happens several times a day with log saying "10/20/2019 11:58:50 Received event "session_deactivate" for unknown device Data".
I've asked VF to send a new router over, but they're being useless as always. It's been 2+ weeks and router hasn't showed up.
If you've plugged a HG612 into the WAN port and you are still getting the issues, then you may as well unplug it (unless you are using DSL stats for your own diagnosis). If you call Vodafone and persuade them to perform some remote checks on the router, these will fail if you use a modem between the router and wall wart, so it would need to be unplugged for these.
*If two different devices suffer the same kinds of disconnects, it usually means that there is a hardware issue somewhere between the cable leaving the modem/router and the cabinet.
*If you live on an estate which has problematic broadband and phone lines, then keep an eye out for OpenReach engineers working on the local cabinet, and chat with the neighbours to see if they have the same issues. **The local cabinet here keeps having inverters blow up, about 2 - 3 per year, and is being constantly investigated!
I had a new router yesterday (the Vodafone Hub Model THG3000) and was getting exactly the same problem for over an hour. Internet would be up for a few minutes max and then it would just reboot with same message in event logs. I ended up putting old router back in and using that.
This evening i spoke via the online chat and was advised to do a factory reset on the router and this appears to have fixed it as it's now been running successfully for almost an hour.