Hi,
Just had the same behaviour with CIC on a new phone (santoni/Pie) and get it working 
In Settings/MicroG/“Google Cloud Messaging” the status was disconnected.
Maybe no app can register in this state ?
I ran the adb root command as described here : https://github.com/microg/android_packages_apps_GmsCore/issues/660
Then conducted a test using a very useful app : https://github.com/bbindreiter/PushNotificationTester_App
After some tries the app managed to register with Google Cloud Messaging, and the status changed to “Connected”
Then I re-installed the CIC app and everything went smoothly
I can now see CIC as a registered app for push notifications in MicroG.
I forgot to mention : SIM with no data access, WiFi only.
Hope this can help !