Unfortunately you can not define a geofence range in the Homey app so if you want to keep using a geofence to set the Surveillance Mode you will have to look at an alternative.
You could try installing the Locative app on your phone and setup webhooks on entering and leaving your defined geofence. Next add some flows to receive and process the webhooks, more information can be found here: IOS Presence app (locative)
(I’m not sure if Locative is available on Android but if not I’m sure someone can mention an Android equivalent)
