Today I tried opening TV Live with my phone, and tablet, right next to the Humax box. And they both connected and worked! Having made the connection, it now seems to connect from other rooms in the house.
The only way to change channel, once selected, seems to be to exit the app and then restart the app. Not too difficult, but not ideal.
However, I also tried the Media Player app. This works on my phone, but the tablet gets stuck on the Welcome page, and I can’t navigate off it.