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I was trying to find the answer myself and just about gave up when I accidentally stumbled upon a fix. What I did was I unchecked the "Fullscreen" option, then dragged the window over to my main monitor, then went back to the main menu without saving the settings. Since I didn't save the windowed setting, it then went fullscreen on my main monitor and I was able to go back to the options and change the resolution to fit my main monitor.

Hope this fix works for you as well!

This worked perfectly! Thanks for the help.