For your reference, here are the methods to solve this issue below:
1. Navigate to the [Mi Home/Xiaomi Home] APP >> select [Xiaomi Smart Doorbell 3S] >> tap the three-dot icon at the upper right corner >> [Notification settings] >> turn on the [Visitor notification] option and [Motion detection notification] option;
2. Navigate to the [Mi Home/Xiaomi Home] APP >> [Profile] >> [Additional settings] >> [Notification settings] >> turn on the [Device notifications] option >> [Device notification management] >> turn on the [Allow notifications] option >> select [Xiaomi Smart Doorbell 3S] >> turn on the [Allow notifications] option;
3. Check whether the mobile phone allows the Mi Home/Xiaomi Home APP to push notifications.