I’m using Armbian based on Ubuntu 20.04.4 LTS.
To fix DNS I added DNS servers to /etc/systemd/resolved.conf. I have two own DNS servers, if you want you can specify nearest appropriate server (for example provider’s DNS or 8.8.8.8, 1.1.1.1, etc.)
...
[Resolve]
DNS=192.168.1.1
FallbackDNS=192.168.1.2
...
Then
systemctl restart systemd-resolved