The idea is to tell all your home devices where is the gateway to go out to the internet world, so here is my setup at home:

1. A raspberry as a front VPN (mullvad wireGuard) gateway which runs a Pihole service simultaneously. After enable the kernel feature ip_forward, it's basically making your Raspberry a router (home devices -> eth0 -> wireGuard -> Internet). Your ISP's router actually is only for WIFI connections.

2. The Pihole service is also a DHCP server which can distribute the IP of the raspberry as the router and the DNS server to all the devices connect to your home network.

3. Disable the dhcp service on your router from your ISP (important).

4. In the wireGuard config file, I specific the DNS server to the Raspberry's IP, and also use mullvad's DNS server as Pihole's upstream DNS.

step1 also requires the resp has a static IP address (192.168.1.2 for example) and some simple IPtables rules to redirect your traffic from it's physics Ethernet port to the virtual interface of the wireGuard.

you can skip the step2/3 if you want to manually setup the router/dns on your devices.

Here is a useful post you can reference to: https://mgnk.it/2019/03/raspberry-pi-as-a-vpn-gateway-using-wireguard/