After installation, both of physical ethernet cable connection and wireless USB connection work well.

After some time both are not working.

To fix the physical ethernet cable connection:

cf.

https://askubuntu.com/questions/1312238/network-interface-down-on-boot


In short:

$ ip l show
The enp5s0 is in the list and its state is Down.
This must be Up.

$ sudo vi /etc/netplan/00-installer-config.yaml
Strangely, enp4s0: is in ethernets.
Change it to enp5s0 as follow:

network:
ethernets:
enp5s0:
dhcp4: true
version: 2

$ sudo netplan generate
$ sudo netplan apply

After reboot, network works well.