That doesn’t work, have tried setting to 192.168.8.2 which is static IP of Pi Hole. Confirmed that static IP is assigned and accessible but DNS isn’t working.
If you use cusomt DNS in gl-inet firmware and force DNS pointing to the Pi, the pi will need to use DoH or DoT to be able to make DNS requests or it will be stuck in loop.
If you are using LuCi and it is not working then that may be down to gl-inet customisations.
A simpler option to test is to set the Pi’s IP address in the client’s WiFi config directly before trying to make it work dynamically.
But why are you using next DNS and pi-hole together? Could you maybe explain a little more what you have and what you are trying to achieve…