Hi,
Please check:
1 - Aqua Mail settings -> Mail receiving.
If you see a green panel / prompt to exclude the app from battery optimizations (near the top of that screen) - please tap the prompt and confirm.
2 - While you're on this screen:
The "Scheduled sync" item should be enabled and "mail check interval" below should be set to something reasonable (the default is 15 minutes).
3 - Phone Settings -> Device Maintenance -> Battery
This screen has a three-position control under "Power saving mode". You'll want it to be set to "off" and then please scroll down and be sure that Aqua Mail isn't listed in "always sleeping apps".
Thanks,
Nadia