sudo cat /etc/resolv.conf
There should be a few name servers listed in there, local, default or Google/Cloudflare.
If there are none, you can add them manually:
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "resolvectl status" to see details about the actual nameservers.
Do note that changes to this file are only temporary, to permanently set alternative DNS servers try this: https://www.ricmedia.com/tutorials/set-permanent-dns-nameservers-ubuntu-debian-resolv-conf