@apostolakisl
Fans are very tricky to support with Alexa. Some may have 3 and others 4 speeds. Plus, when setting the value, each speed have their own thresholds. (Example, 25+ is med, 50+ is med/high, 75+ is high... etc)
Regardless, Fan support is implemented in portal, but it may or may not work with your fan. Add the device, and set the Alexa Category to "Fan". If it works for your fan, then it's great. Otherwise variables is the way to go.
Insteon fans should work, and at least one ZWave fan that I know of - can't recall the model.