I do not get arrival or departure notifications

Check, in order:

The place is shared in the circle by the person concerned, and you turned notifications on for that place (the option “Be notified when a circle member arrives at … or leaves”).
Location is set to “Always”, not “While using the app”. On iOS that choice is offered a second time a few days after install: refuse the downgrade.
On Android, battery optimisation is off for Fenzly. Some manufacturers (Samsung, Xiaomi, Huawei, Oppo) kill background apps regardless: add Fenzly to the protected apps in the manufacturer’s settings.
The other person’s phone has network. A member with no connection for several hours is flagged on the map.

All four points must hold on both phones: the one detecting the arrival and the one receiving the notification.

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