A few more hooks:
Switch your router's upstream DNS to *not* to be Pi-hole (click for details).
You have configured your router both to distribute Pi-hole as local DNS server (via dhcp-option=6) as well as your router’s upstream DNS server (via WAN settings).
The latter defines the target DNS server your router will forward any DNS query it receives, as commonly (i.e. without Pi-hole) your clients would aks your router for hostname resolution.
At the same time, you have configured your router to be used as Pi-hole’s upstream DNS server - thereby creating a loop: Pi-hole and your router will forward the same DNS requests between themselves endlessly (or until time-out).
Clients that have been updated via DHCP to use Pi-hole as DNS server (instead of your router) will query Pi-hole directly, avoiding the loop.
However, updating might occur only on lease renewal, which would possibly leave some clients to use your router until their lease expires after 1,440 minutes.
Find out whether there is an DNS option on *Basic>Network page*, and how that does relate to your `dhcp-option` and/or your WAN DNS settings.
Any additional local DNS servers might interfere with Pi-hole’s operation.
Pi-hole does rely on being the only local DNS server in your network.
On your DHCP / DNS Server (LAN) settings, find out what *Use internal DNS* means.
Again, any additional local DNS servers might interfere with Pi-hole’s operation.
To help with that, find out which DNS servers are in use:
From a Windows client, open a command prompt and execute the following:
ipconfig all | find /i "server"
You want to verify that this lists only Pi-hole’s IP addresses as DNS server.