Access to all HomeKit devices when you are not connected to your home Wi-Fi network is via the iPad, Apple TV or HomePod that is configured as your Apple HomeKit Hub.
If you do not have an Apple device in your home that's set up as an Apple HomeKit Hub, you will not be able to access your devices outside your home, or use automations. Please follow the instructions in Apple's support article HT207057 to set up an Apple HomeKit Hub.
What to do if you still have issues
If you are using a mesh network like Eero or Google Wi-Fi, or a Wi-Fi range extender, your Starling Home Hub (and your Apple HomeKit Hub, if connected by Ethernet) must be connected to your gateway router, not a satellite router or range extender.
If you have multiple HomeKit homes, make sure that each Apple HomeKit Hub is associated with the home in which it is physically located. On Apple TV, the associated HomeKit Home can be set in Settings, AirPlay and HomeKit.
If you are sure your Apple HomeKit Hub is configured correctly but you still have issues when outside your home, please contact Starling Support and include in your email a screenshot of your Apple HomeKit Hub configuration, which can be found via the Apple Home app, 'house' icon top left, Home Settings, Home Hubs & Bridges in the HOME HUBS section.