Thanks for your answer!
I think it is the only interface in WAN. but WAN and WAN6 are configured equally. That means I gave them both the login data and set custom interface to eth1.7. See screenshots:
When I look at the system log, I see the following:
Thu Aug 16 13:25:01 2018 daemon.notice pppd[10336]: pppd 2.4.7 started by root, uid 0
Thu Aug 16 13:25:13 2018 daemon.warn pppd[10281]: Timeout waiting for PADO packets
Thu Aug 16 13:25:14 2018 daemon.err pppd[10281]: Unable to complete PPPoE Discovery
Thu Aug 16 13:25:14 2018 daemon.info pppd[10281]: Exit.
EDIT: I thought maybe the ISP is blocking me due to malformed connection attempts in the beginning. But waiting an hour and trying again did not solve the problem. I flashed the AVM stock image back to the FB4040, restored the settings and internet is working again. I captured the PPPoE traffic. There, the PADI request is regularly answered with PADO and the protocol flow continues. Don't know what's wrong with my OpenWrt setup.