One of the most frequently encountered issues for new Signal users are the delayed notifications. There is an official article for troubleshooting notifications and Signal is also actively collecting user feedbacks with which they can find the root cause of the problem for the different devices. Unfortunately, custom ROMs are out of scope and the effort is restricted to stock Android ROMs.
When I installed LineageOS 18.1 for the first time, I realized that Signal notifications are only received when the screen is turned on. I turned off battery optimizations and background restrictions for the application, but the problem still persisted.
After checking the logs it turned out that the problem is related to Android’s Doze. Even though the background restrictions were disabled for the application, it was not whitelisted for Doze.
You can check the whitelist on your device by executing the following command:
adb shell dumpsys deviceidle whitelist
If you cannot see
org.thoughtcrime.securesms on the whitelist, odds are you can fix the notifications by following the solution below.
You can add Signal to the whitelist by executing the following command:
adb shell dumpsys deviceidle whitelist +org.thoughtcrime.securesms
This command will disable Doze for Signal, and the notifications should appear immediately regardless of the state of your screen.