There are many variations of these USB routers and some work better than others.
When you first plug them in the little blue light on top should flash, indicating that they’re in pairing mode. You then pair them with SkyConnect in the usual way.
You can force them into pairing mode by powering them on and off five or six times. If you’ve got them plugged into some kind of USB charger, my experience has been that you have to actually pull them out of the socket to power off.
Yes, they should appear in the mesh diagram.
Lots of good advice here.