If you receive the message “cannot refresh cache whilst offline” from with Cockpit -> Software Updates, it’s likely due to networkd being used as renderer in Netplan. This issue came up running Ubuntu Server 20.04 and was resolved by switching renderer to NetworkManager.
Edit Netplan .yaml file, located in /etc/netplan. In this case, default was /etc/netplan/00-installer-config.yaml
By default, renderer was set to “networkd”, changed to “NetworkManager”. Simple example .yaml below:
network:
ethernets:
enp2s0:
dhcp4: true
version: 2
renderer: NetworkManager