Have a nice day.
I suppose my latest problem is very embarrassing, so perhaps noone wants to help me?
On the other side, it should be flat basics of OpenWrt, hence most member should be able to solve it. Would someone please take pity on me?
OpenWrt status looks like that (5 screenshots):
(alternatively: https://picload.org/view/dlwiiccl/openwrt_1.jpg.html and the following 4 pictures).
I suppose I have to convert one of the 3 "interfaces", probably "LAN", perhaps one of the "WAN"s, into PPPoE or PPP:
Edit -> select desired Protocol -> Switch Protocol -> enter both PAP/CHAP username and password from my ISP -> leave other options open (or enter sth.) -> Save & Apply.
I have not been around the weeks following, thus everything stayed the same.