A common mistake allot of the fresh hackers make is mistakenly assigning Pi-Hole's IP address to their modem/routers upstream DNS resolving path.
Depending on router/modem, this setting is often called:
WAN settings (not to be mistaken by WLAN setings);
Internet settings;
DHCP client (not to be mistaken with DHCP server);
or "upstream" DNS something.
You dont need to touch these setting ... leave it as is.
If you have not customized allot of settings on the modem/router, its pretty safe to restore the modem to factory default settings.
All you have to do is find the DHCP server section on the router/modem and tell it to hand out Pi-Hole's IP address to the clients as being a DNS server for resolving names to IP addresses.
Have a look at below thread where it turned out the router wasn't able to do this: