Family Sharing members cannot access Home remotely

Having a weird issue, that previously wasn't an issue but cannot peg the cause.

– My wife and I have a family sharing account configured, that has several HomeKit devices registered in Home.

• We have an Apple TV 4K configured as a HomeKit hub, with my iCloud account as the primary account. She has an account on the Apple TV via Family Sharing, which has iCloud, iTunes and Game Cloud configured to her account.
• Remote access is enabled on the Apple TV, and remote access is permitted to her account in the Home app.
• Both of us can control devices whilst connected to the Home WiFi network.
• Only myself can control devices whilst outside of the Home network, everything else is "unavailable" to my wife.

I have attempted to remedy this by:

1. Removing her from the Family Sharing Home and re-inviting.
2. Removing her from the Apple TV and re-adding via Family Sharing.
3. Toggling Apple TV "Use as a HomeKit Hub" and her iCloud "Home" toggle on all her devices during the dance above.
4. Turning off all her devices, and the Apple TV during the dance above.

And yet the issue persists. All devices are running the latest versions of iOS, tvOS.

To make things a bit confusing, the Apple TV sits in an "IOT" VLAN that has limited access, including blocked access to UPnP on the router – however, as mentioned, this does not cause any issues with me for remote access.