Android disappears when tethering

Hi... struggling a bit!
I've just upgraded from a Xiaomi Mi A2 to the A3.
If I plug my phone in to my Asus AC87U then it is detected, and shows up that a Mi A3 is connected to the USB 3 port... however, as soon as I turn on tethering, it disappears and doesn't detect the device anymore.
If I turn tether off again on the phone, it detects it again straight away.