If Mitti gives you a spinning wheel and stays in “Connecting...” state, that’s most likely a driver version mismatch between the switcher and Mitti, and you need to download the latest ATEM Switcher update from https://www.blackmagicdesign.com/support/family/atem-live-production-switchers.
If you can’t make a connection between Mitti and ATEM Software Control, in most cases that’s because the subnet mask is different on your computer’s Ethernet connection and on your ATEM. Make sure they’re the same - usually you want to use 255.255.255.0