Perhaps your Threema works fine?
I had to do some steps which you may need to do.If yours is working fine dont worry about it; however ive put below what i had to do just in case you or anyone else encounter a similar issue.
I found this thread useful
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Searched phone setting for micro G, enabled google device registration (apparently MicroG anonomizes the use of the service), and then enabled cloud messaging.
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I had to download Push notification tester from the /e/OS app store
3.On running it i had a red cross next to “register for push notifications”
- i went into the phone dialer of the regular phone and entered
*#*#CHECKIN#*#*the letters are actually numbers 2432546 be sure to pay attention to the stars, hashes, as after 6 (N) it is now hash, stars to enter.
5 ran the push notification tester and got all green ticks.
6 went into threema about>trouble shoot and reset push token
7 searched for MicroG in the phone`s /e/os settings and now see that the apps are registered.
hopefully now works:)