Well, here really seems to be something odd with connman but first check in
/var/lib/connman → the related service directory
the content of the settings file.
It should contain AutoConnect=true and the Passphrase= as well.
Again look at the how-to article [HowTo] Configure Tvheadend 4.2 and the Sat>IP Server AVM FRITZ!WLAN Repeater DVB-C on OSMC
and look for the service /etc/systemd/system/ip-alias.service defined there as an example for you.
In your case your service should contain the ExecStart for specific connmanctl commands
/usr/bin/connmanctl scan wifi
/usr/bin/connmanctl connect <your wifi service>
In theory that should do the trick.