If the engine isn't detecting the refresh rates supported by your monitor/display you can override it with this launch option:
- Right-click Unturned in your Steam Library
- Select Properties...
- Find the Launch Options field
- Type "-refreshrate=#" without quotes, replacing # with the desired value e.g., 60, 120, 144, etc.
Note: the refresh rate is only used in exclusive fullscreen mode. (not windowed or borderless fullscreen)