I would advise including a debug log to be sure this is the cause, but usually this happens when the router's WAN DNS is configured to use Pihole and all the LAN clients are configured to use the router for DNS.
DNS query comes from a device and goes to the router. The router sends the DNS request to Pihole; as far as Pihole knows, the client asking the question is the router.
To correct this, one typically needs to find the LAN configuration settings under DHCP in the router, and define the Pihole IP for LAN DNS resoluion.
I tried DHCP server on pihole. However, there were some problems. I had explained it in pihole reddit but let me paste it here.
PiHole DHCP server after 10-12 hours losing all connection on devices
Hi,
I have set up DHCP server for my selfhosted raspberry pi on pihole. However, after 10 - 12 hours randomly all connection gets lose. None of the devices can access even local internet.
I have 2 router, 1 is main where isp is connected, the other one extender (where all the wired and wifi devicesis connected)
I have disabled both of the routers DHCP and enabled DHCP on pihole. (picture below about my settings)
My raspberry pi is connected to Extender router (192.168.31.1), i set the range between 5 to 175 and set the raspberry pi 192.168.31.2
at first hours its working properly and i can set static ip adresses for devices on pihole.
however,after 10-12 hours randomly all connection gets lost.
Additionally: I have DNS settings on Extender Router as well, which is Pihole DNS.
Second Thing i had tried:
All the settings are the same however, i did not disabled the main router DHCP (where the only extender router connected to) but i had the same issue again.