I have a few sites with the Vigor 120 on the WAN two interface, using PPPoE settings in router. Every so often these modems need a reboot as lock up etc.

I can remotely reboot the main router, but this doesn't always cure the issue, power cycling the modem does.

Is it possible to remotely access the modems admin page when using PPPoE? From WAN1 perhaps?

Not sure but it may let you do if you go at System Maintenance >> Management and enable allow management from the internet ... not sure if needs to connect it via pc to change the settings once at pppoe bridged may not give you access connected to the router?