Hello,
Sorry if the information on the wiki page is not up to date. It should be updated in the future.
You can follow the configuration example, but you will need to download a relay package and then configure the Relayd on a different configuration page.
Please, navigate to Services -> Package Manager and download the Relay package from there.
Also, before you disable the DHCP server (if you disable it before the next step, you will lose access), navigate to Network -> Interfaces -> Relayd and add a new relay configuration there.
To ensure that two-way communication, navigate to Network -> Firewall -> General settings and edit the WAN zone (WAN => LAN). Add LAN zone in Inter-zone forwarding to Allow forward to destination zones section.
Then, you can disable the DHCP server in Network -> Interfaces -> Edit LAN interface.
Let me know if this works for you now.
Kind Regards,
Andzej