Unable to route to Internet on R7800 unless DHCP server is on.

I re-IPed my house with a range: 172.25.100.0/24

I changed the IP address on my router to 172.25.100.1

I changed my DHCP server (a pi-hole) to give out addresses between 172.25.100.100 and 172.25.100.251, with a gateway of 172.25.100.1.

I then rebooted the pi-hole and the router. Then I started to reboot devices.

And that's when the trouble started...

All my devices got a 172.25.100.x address.  DNS was set to 172.25.100.2 (The pi-hole IP) and the default gateway was 172.25.100.1.

With those settings NOTHING could get on the Internet.  I rebooted the router.  I rebooted the pi-hole. I double checked settings.  Nothing worked.

What finally did work, was to turn off DHCP on the pi-hole and turn on the DHCP on the router. Any IP address the router assigned could get on the Internet.

At this point I figured it was a security setting of some kind. So I went into Security→Access Control.  But Access Control was disabled. And devices I configured with static IP addresses could get to the Internet, but only when DHCP was on.

When my router was set to 192.168.1.1, I was able to use the pi-hole as a DHCP server. But with the new private address range, I was unable to do so.