I thought I do that by ssh-ing into the router and type ifconfig? This shows details for archer, br0, eth0, eth1, eth2 and eth3. Was I supposed to use the br0 details?
Wanted to run Pi-hole directly on the Asus RT-AX58U v2 running Merlin so I don't have to hook up a Raspberry Pi to it. I found this which does exactly what I want - install Pi-hole as an Entware package directly onto the router.
The readme shows how to add the repo. And the wiki shows how to...