Vlan tagging and PPPoE in MacOS Big Sur 11.1

Hi 

My Internet service provider requires for my Mac to directly connect to the Internet, to do the PPPoE request inside a VLAN. (PPPoE PADI/ PADO and subsequent Internet traffic must be tagged with this VLAN.) In MacOS “Network preferences” applet I can go to the "Manage Virtual Interfaces" and create the VLAN on the desired Ethernet Interface. Going back to the previous menu, I then have to configure the IP-settings inside the new VLAN-instance. In the pull down menu I can chose everything from DHCP to PPPoE. If I choose per example a manual IP-addressee the interface with VLAN comes up properly and traffic is correctly VLAN-tagged. But if I choose PPPoE from the menu, the “Network preference” applet creates an new PPPoE instance but doesn’t ask for my PPPoE credentials. Obviously the new PPPoE interface can't come up without entering Username and Password. There must be an error. (PPPoE only works correctly when connected to the Ethernet Interface, but then of course the connection is done without the necessary IEEE 802.1Q header). What am I doing wrong, or is there a work around or CLI command?

Best regards

PatRau13