TP LINK TD-W8951ND: Configuring LAN Port as WAN for Radio Internet & Optical Fiber Usage

Hello,
a few years ago I bought an ADSL modem router (TD-W8951ND) because I had internet provided over the telephone line. As you know in this case, the WAN port is RJ-11. After switching to radio internet, to my surprise, it turned out that I did not need to change the router because the installer configured it so that one of the LAN ports worked as WAN. Due to the fact that some time ago I switched to optical fiber, the router became unnecessary for me and it just so happened that I have someone to give it to, so I'm going to do it. The problem is that I don't really know how to configure it the way the fitter did, because as far as I know, an ADSL modem will not be needed. I will be very grateful for any hint
best regards

W8915ND can only work as an AP (access point) will not work as a WAN router - https://forumsieciowe.pl/index.php?/topic/17-router-bbezpieczwiękowy-jako-ap-punkt-dost%C4% 99powy /

Thank you for your answer. So if I set the router as in the guide and connect the internet cable to it, I will achieve the desired effect?

PierwiastekZdwóch wrote:

So if I set the router as in the guide and connect the internet cable to it, I will achieve the desired effect?

Provided that there will be another router somewhere in the apartment (then as the AP this old router can work like that).
If you connect it as described (directly to the cable with the net as AP), WiFi internet will be provided ONLY to the one who connects to the internet first.