I just did a test migration of an Ubuntu 20.04 virtual machine from our on-prem data center to Azure.
The created virtual machine is not accessible on the network at all. Similar Ubuntu VMs (20.04) that I have migrated also have the same problem. When I migrated an Ubuntu 18.04 VM, the networking was accessible without intervention. 100% normal operation.
I have a VPN gateway set up to the VM's virtual network and can successfully ping/RDP to other windows/ubuntu 18.04 VMs that I have migrated.
I am suspecting that NICs or the Hyper-V Linux Integration Services were not properly installed during the migration as outlined here: https://docs.microsoft.com/en-us/azure/migrate/prepare-for-migration
Please let me know what other information would be helpful in troubleshooting this issue.
Thanks!