The current version of Ubuntu Server 20.04 LTS uses netplan in order to configure the network. If it doesn't use cloud-init to provision the network, the network configuration will be in a YAML file in /etc/netplan. There you can change the network parameters, such as the default gateway if you have a fixed connection.

After changing them, run this command to apply them (remember that you may disconnect from the network, so don't run it from a remote connection!):

sudo netplan apply

A more detailed explanation can be found in this very helpful post at linuxize.