App to rotate .mp4 video for Windows or Android
If I've understood you correctly, your video appears upright when viewed in portrait orientation and rotated in landscape. This suggests the image is stored in a landscape matrix but with no rotation tag set. If so, there are two things you can try using ffmpeg, a free command line tool.

ffmpeg -i input.mp4 -c copy -metadata:s:v rotate=90 output.mp4


If your landscape devices don't understand or obey the rotation tag,

ffmpeg -i input.mp4 -c copy -vf transpose=2 -c:v libx264 -crf 20 output.mp4


The 2nd command will re-encode the video portion of your file. CRF controls the quality - lower is better but will produce bigger files.

In the commands, you may have to switch the 90 with -90 and 2 with 1 depending on how the video was stored.