DNS resolution is not available, Rpi Zero 2w, DHCP on router,

[mgrimace]

The issue I am facing:
When updating Pi-Hole and/or Gravity it says 'DNS resolution is not available'. /etc/resolv.conf shows two nameserver IPs #1: this Pi-hole (on a RPi), and #2: my second Pi-Hole (which is an LXC on Proxmox).

If I manually change the resolv.conf nameserver to either 1.1.1.1 or my router/gateway 192.168.0.1 the Pi-Hole does update properly; however, the change doesn't persist on reboot.

What is a proper fix that will persist on reboot?
Details about my system:
I have installed Pi-Hole on my Raspberry Pi Zero 2w. I have a usb-ethernet adapter and it's connected by ethernet cable.
DHCP is handled by my router (it's a TP-Link and won't let me disable DHCP, so I can't hand it over to the Pi-Hole). I have unbound installed

What I have changed since installing Pi-hole:
Adding the second Pi-hole, and performing a manual update of the current Pi-Hole.

[mgrimace]

Solution:
I followed this guide to set permanent DNS nameservers (link)

Steps:

Install the resolvconf package:

sudo apt install resolvconf

Enable and start the resolvconf service:

sudo systemctl enable resolvconf.service && sudo systemctl start resolvconf.service

Open resolv.conf using head file and enter your nameservers:

sudo nano /etc/resolvconf/resolv.conf.d/head

I used 192.168.0.1, my default gateway address, which should then refer back to Pi-Hole 1 and Pi-Hole 2 for DNS (assuming the other Pi-Hole is up/available). I'm not sure if this is the best, but it works.

Update resolv.conf nameservers:

sudo resolvconf -u

Open resolv.conf to confirm new nameservers have been written:

sudo nano /etc/resolv.conf

reboot to confirm it remains