Does it show on iOS that it is charging? Does this alert appear when you connect your iPhone to your computer?
In iOS, the “Trust this computer” alert belongs to privacy, not the network. So if you think iOS needs to be reset, you should go to Settings > General > Transfer or Reset [Devices] > Reset and tap Reset Location and Privacy.
I think you should check the USB port and your cable first. When a powered lightning cable is detected, the iPhone will be charged first, whether the user trusts the device or not.
Does the USB port on the Opal work properly when I plug in another device (e.g. USB flash disk)?
I finally managed to connect my iPhone via the USB port on the Opal with the Tethering function (so at least we know that the USB port is working), but whilst the Opal is connected to WIFI internet at a cafe.
I’m travelling at the moment and will do more testing when I am home. Thank you so much for your tips so far, all very helpful!