Be sure your video stream matches. The display monitor has a "native" resolution, which Windows refers to as "recommended". Match that with the Windows Display Settings. Then match that in the game Graphic Settings on the Options menu. You might have to do an even division of a given ratio if you have a high resolution, such as using 1920x1080. The game cannot do an ultra-wide setting without extra modifications, and if you have one of those, use the "Windowed" choice in the Graphic Settings of the SH4 game.
One of the most common issues with the 'modern' computer and OS, as it relates to old DirectX v9 games, such as Silent Hunter, is the SH4.exe file "Properties". Navigate with File Explorer into the main game folder, and right-click on the SH4.exe file. Choose "Properties" on the resultant context menu. Then choose the "Compatibility" tab and try some of these settings:
The "Disable fullscreen optimizations" or "Change high DPI settings" might be what is keeping your game from starting.
One last thing, when adding mods to your game, is that you want to be certain that you have a "clean" SH4 install, and after activating FotRSU and Nippon Maru, be sure and empty the Save folder ("C:\Users \UserName \Documents \SH4" by default) so that the mods fully activate. That will remove all of your previous progress in the game.