First thing to check, is Pi-hole responding to DNS queries from a client.
Whats outcome from below one on a client PC (can be Windows or Linux) ?
nslookup pi.hole <PIHOLE_IP_ADDRESS>
Next, check if you get the same results if you run the same nslookup but leave out the "<PIHOLE_IP_ADDRESS>" bit.
Whats outcome of below one on a client PC ?
nslookup pi.hole
If results differ, maybe you get an "NXDOMAIN" reply for the "pi.hole" lookup, you've configured something wrong somewhere.
What are DHCP assigned IP details on a Windows client PC when run below one in a CMD box ?
ipconfig /all
*Am only interested in the DHCP and DNS bits.
Or if on a Linux client PC, what DNS server is configured/displayed (DHCP assigned or static) with below one ?
cat /etc/resolv.conf
Below link describes preferred/recommended configuration of the router:
If not possible because particular settings lacking on the router, you could disable the DHCP service on the router and as a replacement, activate Pi-hole's DHCP service instead: