You would set-up the Pi-hole IP in your DHCP server to be handed out to the clients automatically.
On the router that is (if your router is the DHCP server/Hadler).
Then, everything that connects to the network and gets a leased IP, also gets the DNS setting pushed to it.
So all your clients would be set-up as Auto for IP settings on the Network properties.
If your router however (and I fear that this is the case with your device) does not have the ability to push the DNS setting to the clients, you have 2 options:
a. Either set the Pi-hole IP at WAN level (DNS setting), OR b. (recommended) use the Pi-hole DHCP function and disable the DHCP in the router.
When using a., all the queries will show as originating from the router (in the query log on the Pi-hole admin interface) and you won't be able to track who's who doing what.
When using b., you get to see who's requesting what AND your DNS server setting is automatically pushed to the clients.
One thing to keep in mind.
If you're using a secondary DNS (in the router under WAN DNS2), there are chances that queries will bleed through it and hence, bypass the Pi-hole restriction.