DNS broken & SSH not working on Ubuntu 18.04 running Pi-Hole 4

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Expected Behaviour:

DNS queries resolved, able to log in to server via SSH

Actual Behaviour:

Can't log in to server with SSH using IP address and DNS queries on the server itself and via clients do not work.

Debug Token:

Can't get one as DNS on the server itself is busted

More Info

I had a power outage and my Ubuntu server 18.04 was taken down for a few minutes. When it came back up, DNS resolution was broken both for clients, and locally on the server and I couldn't log in to the and I don't know why.

The Pi-Hole web interface no longer works.
When setting my clients to use it for DNS, no queries resolve.
Weirdly, I can't even log in to the server over SSH using the server's IP address (not via a hostname).

I've logged in via the local console and trying to do pihole -r or pihole -g fails as the server can't resolve domain names.

I've manually added my ISP's DNS server via netplan, run pihole -r and done a repair, removed the nameservers from netplan and everything works fine. Until I reboot and DNS & logging in via SSH is busted again. Weirdly, SMB continues working fine, but SSH does not.

Anyone know what's up here? It's doing my head in!

Alright, I think I figured this out. My server has two network interfaces (one for ILO, one for usual network stuff), and in the past, the 1st network interface (ILO) has never received a DHCP address on boot in Ubuntu. But today it did and that seems to be why nothing worked.

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