That might actually be more of a question for Samsung or NAS. It sounds like you've output a file according to their recommended specs. It even works on your Samsung tablet!
So it might be necessary to contact Samsung to find out why .mp4s won't play on your TV.
By the way, were you able to get MP4s output from your Corel program to play on the TV?
Actually, as I re-read your post, I wonder if the problem is your Synology unit.
You say that it's the actual device that's playing the file.
How does the TV use it to play your video?
Or is it the DLNA that's playing your video?
It's just not clear from your question which device is actually reading and playing the MP4 file.
After reading your answers I tried to put a file on an USB-stick, and that works. So I think you are right it might be the DLNA-connetion that is the problem.
I have some files from my old camera, but yhat is SD and coded with Mpeg2 (So; no, my Corel did not export .Mp4). They work fine, and that is why I imidiately suspected the .mp4's.
But your answers have lead me to investigate other things.
Thanks 4 help.
Glad you've found a solution, Settemand. Please be sure to mark this thread/question as answered!
I have investigated further, and now everything works. Thanks. I will close the thread.
Just to follow up.
You lead me in the direction of the DLNA and I found out:
My Synology also works as Squeezebox server, and the latest update installed a some DLNA software that took over the DLNA connection. And that did not support .Mp4.
So I deactivated that, and had to reactivate the DLNA in the Synology system. And now everything works like a champ. :-)