How to convert MP4 files so that Insignia TV can play them

Wow, you'll be using A LOT OF RESOURCES for your watching pleasures I guess. For every hour you want to watch, you'll be working 3 hours on it! That's not how the current technology stands.
But if you really have no budget (please think about the DVD-R that's piling up, you'll break even in one or 2 years with a brand new TV):

1. https://www.mp4joiner.org/en/

Join your mp4's to the max video length of a DVD (just around 2 hours)

2. https://www.dvdstyler.org/en/

To correctly make it a universal video DVD (instead of the file DVD you made earlier), use this program, and drag and drop your final MP4 file and burn it.

As said, the DVD-R discs, the time invested for every burn (time, money to leave the PC's on for hours), may be better spent on a new TV that supports MP4 file through USB. Saving you time and money, and the pleasure to IMMEDIATELY watch what you want.

Likely problem is the audio or video codec in your original .mp4 files.

For example, using mp3 or ac3 or eac3 audio streams fail on most playback units.

Also many playback units will work with 44100k audio streams + fail oddly with 48k audio streams.

Same with surround sound. Stereo always works. Many surround sound encodings produce loud insect like audio artifacts.

Manufacturer specs for Insignia NS-19ED200NA14 TV show a USB port available as a media input.

First get your videos working off USB, so you avoid the wait time of burning a DVD + then throwing away the media when the burn fails or the burned streams won't playback.

Introspect one of you .mp4 files looking at the audio + video stream types.

Here's an example of a video which can play on most devices...

imac> avinfo file.mp4
file.mp4
   length: 00:48:25.80, start: 0.000000, bitrate: 9327 kb/s
   stream: #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 9259 kb/s, 23.98 fps, 23.98 tbr, 16k tbn, 47.95 tbc (default)
   stream: #0:1(eng): Audio: aac (HE-AACv2) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 62 kb/s (default)

Notice the video stream is h264 which will playback correctly on any device. Also notice the audio stream is AAC, which will also playback on any device.

I think that you may be going at this the hard way.  Possibly a better solution is a media player such as the WD TV.  It is available inexpensively on the fleabay and plays any of the common audio and video formats.  Plug a flash drive into it and play from that.  One can also attach a 1 TB USB drive and store weeks of media.

https://www.ebay.com/sch/i.html?_from=R40&_nkw=%22wd+tv%22&_sacat=0

Thanks for all the advice.  As you can tell, I know nothing about this.  Here is the background.:  I am learning to play the keyboard and I downloaded some youtube instructional videos and they are in an MP4 format (don't know how they got to be MP4's).  Anyway, I bought a small Insignia TV which has an internal DVD player and mounted it above the keyboard with the idea that I could watch the video, pause, etc. while I learn.
In looking at the back of the TV, I noticed that it has a USB port as well as an HDMI port.  Are you saying that if I copy the MP4 files to a flash drive I can stick it in the USB port on the TV and it would play the videos?  Also, what format do the files need to be in in order for the TV to play them?
Thanks

1) You asked, "Are you saying that if I copy the MP4 files to a flash drive I can stick it in the USB port on the TV and it would play the videos?"

Yes.

2) Also, what format do the files need to be in in order for the TV to play them?

If you download from YouTube, place the downloaded files onto your USB stick.

They will play as-is.

Burning to CD/DVD... is an entirely different process.

David, I tried your suggestion and the only thing that my TV will play using the USB input is photos.  Not videos.

Kimputer, I tried your suggestion as well but after I burned the DVD it couldn't play it because it was not associated with an app.  I think you are right.  It's not worth it.
Thanks anyway.

According to the manufacturer specs on your TV, you should be able to play videos also.

Maybe post whatever error message you get when you try playing a video off your USB stick. Likely this will be sufficient to suggest some way to use your existing TV.