A client had an .h264 from one of their clients DVRs.
I was unable to get it to play, but using Media Info I found the file type was actually .AVC
I must have tried at least 15 methods of playing and converting before I found this post which linked me to MKVToolNix
After installing and running MKVToolNix,
then simply navigating to the file,
Then clicking "Start Muxing"
which will create a MKV file in the same directory as the original which played flawlessly!