The app doesn't save notifications when the phone restarts because it doesn't run in the background.
However, every time you open the app manually, it saves them and doesn't work properly. 
We have articles for Huawei and Xiaomi phones, but if you are using a different brand phone, you can learn how to do it by looking at these articles. 
Huawei: 

https://apptosd.com/en/article/how-to-turn-on-automatic-startup-for-huawei-devices

Xiaomi:

https://apptosd.com/en/article/how-to-turn-on-automatic-startup-for-xiaomi-devices

Please leave us feedback if you have encountered a different problem.