@ArcherC8 no, I don't think I have. In any way, the question "resolved iself". I gave up on port forwarding and hosted the files I wanted to host on a web-server. Sorry for wasting your time.

@ArcherC8 I couldn't do an address reservation, beacause my MAC address is apparently invalid. I managed to get to the device via the internal IP, but neither the external IP, nor DDNS work.

@ArcherC8 Thank you for your advice, but I will try probably try it tomorrow. It is already quite late in my timezone, and I don't know half of these terms :)

@ArcherC8 Dinamic, but I have switched to static for a few hours, because that is what one site recomented to do. Then, when I couldn't reach the server I switched back to the dinamic IP, but the...

@ArcherC8, thank you for the reply! I already know my public IP, but when I go into Network/WAN Settings, IP displayed there is different from the IP I get using whatismyip.com.

Is it normal? And can it be the reason why I cannot forward ports?