dhclient
is the Dynamic Host Configuration Protocol (DHCP) Client one would use to allow a client to connect to a DHCP server.
$ sudo nano /etc/rc.local
#!/bin/bash
dhclient
exit 0
$ sudo chmod 755 /etc/rc.local
$ sudo systemctl enable rc-local
$ sudo systemctl restart rc-local
$ sudo systemctl status rc-local
I had this same problem with Ubuntu Server 20.04 LTS. The fix works, so you are a deity, and you have my sincerest gratitude - but WHY do I need it? I have other VMs that are running Ubuntu 20.04, that don't need this fix. They needed IPv6 turned off, to avoid the 2 minute boot delay, but that was it. DHCPv4 works fine.
Thanks!