The WeMo dimmer switch is going to be tricky. It is not officially integrated with smartthings, although some other Wemo devices are. Again, “the model number matters.“
There is a community work around that some people have been successful with it, but it does require installing custom code, it’s a little tricky, and some people have said they had to try it multiple times before it worked. So I don’t know if you want to go to all that trouble or if you want to switch to a different brand that works better with smartthings. (Meross is a good WiFi switch available in many regions and has a good integration with smartthings for most of their models. It also typically costS less than WeMo. 
)
If you do want to try to get a Wemo dimmer to work, see the following thread:
And if you’ve never used custom code with smartthings, here’s the FAQ for that. The topic title is a clickable link.