Unable to get networking working after changing motherboard

Networking in Ubuntu server 20.04 and later is handled by netplan. Your netplan file refers to the ethernet interface eno1. Since you moved the SSD to a different motherboard, the interface is actually now enp5s0. We’ll simply change the netplan file to match.

First, we need to know the exact name of the file. From the terminal:

cd /etc/netplan && ls

This will show the exact name of the file. Now let’s edit it to make our change:

sudo nano fileyoufound.yaml

Change this file:

# This is the network config written by ‘subiquity’
network:
  ethernets:
    eno1:
      dhcp4: true
  version: 2

To this:

# This is the network config written by ‘subiquity’
network:
  ethernets:
    enp5s0:
      dhcp4: true
  version: 2

Netplan is very specific about indentation, spacing, etc., so proofread carefully twice. Save (Ctrl+o followed by Enter) and close (Ctrl+x followed by Enter) the text editor nano.

Follow with:

sudo netplan generate
sudo netplan apply

Did you connect?

ping -c3 www.ubuntu.com