Previously I just edited /etc/network/interfaces (or added file to interfaces.d), but Ubuntu 18.04 uses Netplan now. The old way worked by editing the SD card, so you never had to login into the serial console. I'm not sure if there's a way to do this with Netplan, but this is what I did to get wifi working:

• Login via serial console or plug in Ethernet (I used the Duo mini shield which has Ethernet port)
• sudo nano cat /etc/netplan/armbian-default.yaml
• network:
    version: 2
    renderer: NetworkManager
    wifis:
            wlan0:
                    dhcp4: no
                    addresses: [192.168.1.69/24]
                    gateway4: 192.168.1.1
                    nameservers:
                            addresses: [192.168.1.1,8.8.8.8]
                    access-points:
                            yourssid:
                                    password: yourpassword

• sudo netplan generate

• sudo netplan apply

• ip a

Make sure to modify the addresses, gateway4, nameservers, access-points to fit your needs.

Armbian Bionic stock settings are set to Network manager override:
https://github.com/armbian/build/blob/development/lib/distributions.sh#L278-L282

/etc/netplan/armbian-default.yaml
		network:
		  version: 2
		  renderer: NetworkManager

... which means the default/preferred way of connecting to the wireless (nmtui-connect) works out of the box.

 

On the other hand, Netplan is quite handy.

OK, I'll try nmtui on the next Duo I configure to compare the two.