There is a "Lock Mouse" setting in the options which is intended to toggle whether the cursor gets locked inside a non-fullscreen window. I'm not sure if that will do anything to help. I have seen screenshots of people playing the game on multiple monitors but I doubt they were different resolutions. You may want to google for multi-monitor mixed resolution in other games using the Unity engine.
In the meantime, I found what I think is a way of easily adding more explicit support which isn't already in the game (https://docs.unity3d.com/2017.4/Documentation/Manual/MultiDisplay.html). If you are up for it, you can test out the change since I don't have a multi monitor setup at the moment.