This question also related to the following questions:
Why does the app not working sometimes?
Why does the app send message with a delay time?
Why does the app only send message once I turn screen on or open the app?
Many phone manufacturers customize the android operating system on their phones to prevent the app to run from background.
To get it working without restriction, we recommend checking out the links below and follow its instruction to
enable auto-start permission for Do It Later on your device
Pixel: https://dontkillmyapp.com/google
Huawei: https://dontkillmyapp.com/huawei
Samsung: https://dontkillmyapp.com/samsung
Oneplus: https://dontkillmyapp.com/oneplus
Xiaomi: https://dontkillmyapp.com/xiaomi
Oppo: https://dontkillmyapp.com/oppo
Meizu: https://dontkillmyapp.com/meizu
Asus: https://dontkillmyapp.com/asus
Realme: https://dontkillmyapp.com/realme