Was this a unity game? (Sorry, can't remember and I'm not at my desk at the moment)
If so there are two things you can do to see what it looks like without the need of a new monitor.
- in the game view set the resolution to be custom and try (3440×1440). I labeled mine (UWQHD)
- Try updating the anchor points of the ui to be fixed to the corners of the screen and then move it from there. Leaving things anchor to the center and moving them outward tends to make the ui go off screen of certain resolutions.
If this was Godot or Construct or GameMaker or something else then sorry for useless advised ;)