Pairing Tuya Zigbee devices

I’m trying to use the Tuya Zigbee radiator actuators and temperature/humidity sensors to control the heatring in my home. I am trying to pair the devices with the app on my smartphone, and having no success at all. Can anyone help me? I have the TuyaSmarth app on my Android phone. I think I have successfuly got it to recognise the Tuya gateway, which is curretly showing a steady red LED indicator. But I can’t get any further. Basically I can’t see how to put the devices into pairing mode. The sensor has a small button on the top. If I press the button, a blue LED flashes once, briefly. If I long press it, the blue LED gives 3 or 4 short flashes, then one long one, but it’s not discovered by the app. The radiator actuator has touch buttons, but I have found no way of getting it into pairing mode (when it is, according to the handbook, supposed to show a wireless icon).

Can someone give me some hints on how to get these two types of device to pair with the app/gateway?

Another question: Do I actually need the Tuya gateway and the Tuya app to get these devices integrated with OpenHAB? Why can’t I do it just with a Zigbee USB dongle for my computer? What does the gateway do that could not be done by some software associated with the Zigbee dongle?

Many thanks - Rowan