There was some TV makers that broke video playback at just over 2 hours on purpose to stifle video piracy. This sounds like your smart TV has this video playback limited to 30 minutes. That discussion is on the web so I won't write at length about it.
My fix was to not use the Smart TV apps and player but use anything else. Today I use an Intel NUC for playback. There are other devices that can do this too (your PC for example.)
I have a router/modem without Internet service to connect Sony smart TV to Seagate Personal Cloud external hard drive. I have a ton of .mp4 files on the hard drive. I can find the mp4 files on the tv, but about 30 minutes into the movie, the TV says the file is unreadable. The file is fine...I can watch the whole thing on my laptop. Any suggestions to make this work? Is it the WiFi connection?