I have an unused Raspberry Pi 4 (2GB model) lying around and I would like to install OpenWRT on it and use it as a router. I get Internet from DSL so I can’t hook it up on the Raspberry Pi directly, I need to plug an Ethernet cable coming from my actual router to the Pi.

I am no expert on networking, so please forgive me if I say something that is wrong. I want the WAN coming in from the router from the Pi’s Ethernet port, and the LAN coming out as Wi-Fi. I may also stick an additional Ethernet adapter to it in the future. I have tried doing this many times and have failed. So, could anyone explain to me how could I do this?

Also, what are VLANs, what are their uses and if I wanted one, how could I setup it in OpenWRT?

Thanks in advance.