• Open Steam
  
• Go to your Steam Library
  
• Right click "Dungeon Defenders 2"
  
• Hover over "Manage >"
  
• Click "Browse local files"

The "Dungeon Defenders 2" folder should now be open

Inside your "Dungeon Defenders 2" folder

• Open the "DunDef2" folder
  
• Open the "UDKGame" folder
  
• Open the "Config" folder
  
• Open "DefaultDunDef.ini" with Notepad or other Text Editor
  
• Inside of the "DefaultDunDef.ini" file, scroll down to the "[UDKGame.DunDefViewportClient]" section
  
• Under that section there will be 15 ish "SupportedResolutions[_]="___x___""
  
• Pick one and replace the resulution in quotes with your desired reslution

For example I replaced: SupportedResolutions[15]="3840x2160" with SupportedResolutions[15]="3840x1600" because thats my mononiter's reslution.

If you did it correctly when you launch the game and open the options menu the drop down selector should now list whatever resolution you added