hey,
after trying for hours yesterday and also spending a few hours today, i deciced to connect my old router (tp-link wdr3600 with OpenWrt 18.06.1) and set it up like this
and pppoe started to work on FIRST try.
so i wonder why im not able to get it to work on my zyxel nbg6617... i also wonder why WAN is missing on the switch on the zyxel.
i found this thread: VLAN configuration for Telekom
which brought me to the idea, to try another router, because my zyxel and this one are very similar.