Yes, I disabled the wifi on the laptop before connecting, and the laptop gets an assigned IP.
The tests were done after I performed a reset to default settings on the router, so I configured only the PPPoE connection and the wifi (SC on/off, does not matter). The PC gets an IP, ping works ok, so it's not the PC/laptop which is faulty. I tried manually assigning an IP address to the PC / laptop, same problem.
So if I have two devices that work on their own but do not work when plugged in the ethernet port of the router, and also work plugged into another router, I guess it is the first router which has some problems.
Basically, you turn on the router, configure PPPoE from the PC, save the settings, and the internet does not work on the PC. It's simple as that, I don't know how they kept it for a week and then told me it works. Boy I would like it to work, otherwise I would not have sent it away.