Ubuntu core uses “netplan” to setup networks. To add your first wireless network

Modify /etc/netplan/50-cloud-init.yaml by

sudo vi /etc/netplan/50-cloud-init.yaml

By default it will probably have an enthernet config eth0 and you can add your own wireless wlan0

# This is the network config written by 'console-conf'
network:
  ethernets:
    eth0:
      addresses: []
      dhcp4: true
  version: 2
Since I wanted to enable wifi, I updated this file to this:

# This is the network config written by 'console-conf'
network:
  ethernets:
    eth0:
      addresses: []
      dhcp4: true
  wifis:
    wlan0:
      access-points:
        <access point name>:
          password: <password>
      dhcp4: true
  version: 2

Run

sudo netplan generate 

to generate the necessary backend configuration for systemd-networkd.

Run sudo netplan apply to actually apply the new config. You can run networkctl periodically and you'll see the interfaces come up. or run sudo apt update to check if your package manager can connect to the internet