Push notifications for NoMachine 2FA (see: https://www.nomachine.com/support/documents/activating-two-factor-authentication) require that:

i) the mobile device supports receiving push notifications

ii) push notifications are enabled in the device settings.

Remarks for Android devices

Push notifications for 2FA require Google Play Services, which is included on most Android devices running Android 6.0 or later.

Some custom Android distributions like GrapheneOS, LineageOS or CalyxOS don't include the Google proprietary background services.

Push notifications may not work reliably also in case of devices with aggressive battery-saving settings. A possible solution is to whitelist the NoMachine application in the battery settings,  to do that verify instructions for your Android distribution.

In some cases, the device configuration is blocking notifications, verify this in Settings, options like 'Do not disturb' may restrict notifications.

Remarks for iPad/iPhone

Go to Settings > Notifications > NoMachine and ensure that notifications are allowed and the style (banner/alert) is enabled

Verify also that Focus / Do not Disturb modes are enabled, NoMachine is in the allowed list.