I am prepared to concede that I might not have configured the 2820Vn correctly and have therefore made several screen dumps available at http://www.cyberpictures.net/tb to aid diagnostics and hopefully feed back of what needs to be changed.
I'd be inclined to disable both the PVCs on this screen
to see if that silences the DSL reboot message.
If that doesn't work, I'd re-enable WAN 1 on this screen
, change the "Active mode" setting to the one nearest to Off that you have, and disable WAN 1 again.
If that doesn't work, I'd check everything is disabled in the MPoA tab on this screen
It would be interesting to know which of these (if any) works, not that I'm ever likely to use a Draytek router.
I was totally unaware that the DSL socket could be configured as a LAN port but even if that was so, it won't accept a RJ45 plug!
It can't. You have two WAN ports - WAN 1 is an ADSL modem, and WAN 2 is an Ethernet WAN port.