Homekit with iPad as Hub issue

Hi,

I've just installed iOS 10 on my iPhone 6 and iPad Air 2 and try to play with HomeKit for controlling Hue lights remotely.

I can control accessories from my iPhone and my iPad when both are connected to my Wifi network.

I've setup my iPad as an HomeKit Hub thru Settings/Home.

My iPad is well detected as HomeKit hub on my iPhone by the Home App.

The problem is when I disconnect my iPhone from my Wifi network and get connected thru 4G: I can no more control accessories: my iPad is still seen as homeKit Hub by my iPhone, but I get failures when trying to switch lights On/Off.

I've disconnected my iPad from iCloud account and reconnected it (same iCloud account as on my iPhone).

I've disabled iCloud/HomeKit and reenabled it on my iPad.

I've disabled Home/Hub and reenabled it on my iPad.

I've rebooted my iPad and my iPhone.

Verification with 2 factors is enabled on my account.

I've disabled and reenabled iCloud keychain.

I've plugged my iPad on a charger.

No success, any clue?

Could it be linked to my ADSL box settings (UPnP is on)?

Thanks.