Hi,
The issue could be outside the app and should be able to get solved in the phone settings.
Android 6 and newer and its “Doze Mode”:
Doze Mode is where Android will not let apps run in the background as often as they need to while the phone is stationary.
To check, please bring up AquaMail settings -> mail, receiving.
If the app is not already whitelisted, there will be a green panel / button / message / prompt asking you to do this.
I'm not sure if this is the root cause but it's worth to check the system settings.