I use DNS on both routers and saw an option on the RUT to enforce use of its own DNS, even when another DNS (i.e. the Fritzbox WG master) is running. I’m not sure if I should enable or disable it.
Background info:
My Fritzbox is at home and is my gateway. To make things a bit complicated ( 
), I have a Pi-hole with unbound running as DNS on a virtual machine (Synology NAS). The Fritzbox is set up to use Pi-Hole upstream and downstream, i.e. all my clients learn from my Fritzbox (that still is the DHCP server) that they get their IP resolves from the Pi-hole (not from the Fritzbox).
My RUTX11 is in my mobile home and connected to my Fritzbox via WG (physically via WiFi at home when near enough, else via GSM/LTE or public WiFi).
So I have two different IP ranges (Fritzbox: 192.168.178.0/24, RUTX11: 192.168.11.0/24).
When at home, all clients are on the Fritzbox network (except a Raspberry Pi 4 that is connected on the car via LAN to the RUTX11).
However, when on the road, at least our mobile phones and tablets connect to the RUTX11 as its SIM has the better data plan.
Sometimes, I get a bit confused about the different IPs the devices get (abroud vs. home), but that’s ok.
But I’m really lost about many settings the RUTX11 offers, e.g. about DNS, hence my question. 
I have entered the most important clients (mobiles and tablets also on my RUTX11, so they get their IPs dynamically (but always the same) from the RUTX11’s DHCP IP range.
E.g., my phone gets 192.168.178.22, when on the Fritzbox network; and 192.168.11.22, when on the RUTX11 network.
Does it make sense to you?