Decision tree

First, you must log in to the app, then click on the profile icon on the top right corner and choose Push notifications.

Here, you will see several types of notification. If all your notifications are switched off, you can enable them and press Save. 

However, if all your notifications are active, then you should take the following steps: 
1) Switch off all notifications and press Save; 
2) Exit the app and close it completely so that it doesn’t appear in your list of active apps. 
3) Reopen the app and repeat the first two steps, this time enabling notifications and pressing Save. 

When you open the app again, notifications should work. You can check this in several ways: 
1) By transferring money between your accounts, for example, one cent. 
2) By going to a store and paying by card or phone.

Sometimes, Citadele app notifications stop working if notifications are blocked for the whole device. To reinstate them, go to Settings → Notifications. Find the Citadele app in the list and select it. Ensure that all notifications have been enabled.

Source: https://support.apple.com/guide/iphone/change-notification-settings-iph7c3d96bab/ios

Also make sure that your device is not on Do Not Disturb mode. Go to Settings → Do Not Disturb. There, you will see whether this mode is activated.