So thats kind of clear and not clear at all. But it is what it says.
Either the file can not be accessed on the network, or its not a format that plays in a web browser. Some MP4's are more equal than others.
If you just paste that URL into the browser address bar in Chrome, does it play back or do you get an error? If its an error, then that would indicate the URL can not be resolved for some reason. If its a player or it tries to download the file, then the URL is ok.
If it plays back the plot thickens. This could yet be a format thing, or it could be CORS thing.
Lets remove some variables. This is a link to an MP4 file I use for testing.
https://drive.google.com/file/d/1cLyRcALATWiVTt7YW0PX3ewk70o6durT/view?usp=sharing
Download this and put it on your NAS server. This will have no format issues and should play back just peachy, if the URL resolves ok and if there are no CORS issues.
And it just has to be said: 500MB - 1GB is no good. They should be converted to an optimal size and/or format for the web. If the videos are really really long, they should be split into multiple shorter videos. Something like Plex might stream and encode it for you on the fly I suppose. But otherwise I think you should re-consider your strategy perhaps.