Activity Zones Solutions
I've discovered that Activity Zones created using the web interface do not function, or cannot be applied to a custom mode, or the system does not recognize them, or all three. It is my recent and fairly exhaustive experience that the Activity Zone(s) must first be created in the app, and that the custom mode must also be created in the app. I'd suggest creating the custom mode first then at the "In The Following" stage create the activity zone(s) once prompted.
Here are instructions for creating the custom mode: https://community.arlo.com/t5/Arlo-Q/activity-zones-completely-ignored/m-p/1619853#M7438
I think it is possible to edit the activity zones on the web interface without issue after the set up above has been completed. Frankly, I am still exploring this.