If you didn't opt for a static IP during installation, you need to configure a fixed IP address / DHCP lease reservation for your Pi-hole machine in your router, and you have to use exactly that IP address in your router's DNS settings.
Pi-hole will show the IP configured at the time of its installation.
The following command will list IPv4 addresses as currently assigned when run from your Pi-hole machine:
ip -4 address
If that doesn't match an IP address from Pi-hole's UI, please run
pihole -r
and choose Reconfigure to make Pi-hole aware of its changed IP addresses.