how did you create this image exactly ? normally a configuration tool (console-conf / subiquity) comes up on first boot, where you need to register the device with a user account and configure the network settings. do yoou not see this tool ?
With the bluez package the bluetoothctl tool gets installed, you can use it like below to turn on bluetooth and disover/connect devices (below it finds an old Ubuntu phone lying on my desk).
ogra@dragonboard:~$ sudo bluetoothctl
[NEW] Controller 00:00:00:00:5A:AD dragonboard [default]
[bluetooth]# agent on
Agent registered
[bluetooth]# default-agent
Default agent request successful
[bluetooth]# power on
[CHG] Controller 00:00:00:00:5A:AD Class: 0x100000
Changing power on succeeded
[CHG] Controller 00:00:00:00:5A:AD Powered: yes
[bluetooth]# scan on
Discovery started
[CHG] Controller 00:00:00:00:5A:AD Discovering: yes
[NEW] Device 43:45:C0:00:B3:E7 Pro5 Ubuntu Edition
[bluetooth]#
There is an interface shipped by the bluez package you can connect an app snap to, to manage connections etc (called bluez:service).