Rebooting a route - VDSL WAN connection doesn't come back up, until you reboot the modem as well?

We have a WAN connection that comes in via a VDSL modem. (Billion BiPAC 8900X R3). The modem is configured in bridge mode.

Router is an EdgeCore SAF51015I running VyOS 1.4.

The WAN connection uses PPPoE with a username/password.

One strange thing I noticed - when I rebooted the router, when it came back up - it didn't have an internet connection. (As in, the actual Ethernet link to the bridge-mode modem was up - but there was no internet access).

I then rebooted the VDSL modem, and the internet connection came back up.

Has anybody got any theories as to why this might be occurring? Any ideas how to fix, or work around the issue? (I suppose we can get some kind of relay for the modem power - but that seems a bit hackish, for a business application...although open to ideas).