This is sadly a ubiquitous problem with FOSS phone software. Google's and Apple's notification systems are anti-FOSS. You can use your own on Google phones, but

This is sadly a ubiquitous problem with FOSS phone software. Google's and Apple's notification systems are anti-FOSS. You can use your own on Google phones, but then your app will have to wake up periodically to check it, and the system will detect your app as a battery waster, tell the user your app is a battery waster, and automatically prevent your app waking up to prevent battery waste. And on Apple I believe you simply can't do that because they user has to open the app to wake it.