Change DNS server URL - to Pi-Hole server?

[gotLL]

Hello All,
I have Linux Lite 3.8 on an older 32bit laptop
The Ubuntu version is 16.04

I've recently built a Pi-Hole system
When I select "Network" in the Linux settings - I can change the default IP address to my PI-hole address - and the system works.
But as soon as I restart the OS - it reverts back to the default IP address.

Can anyone tell me what file I should update with my PI-Hole's IP address - so that it becomes the default DNS IP address?
Sincere thanks!