Media Offline when using drone videos

I’ve created many DJI Phantom 4 Pro drone videos that playback fine in any media player like MPC-HC or VLC. But when trying to use them in DaVinci Resolve I immediately get only a Media Offline message (see below). I don't know why.
The drone videos are 4096x2160 with 99,997kbps bitrate, 29.970fps MP4 format, H.264.
This is a fresh install of the free version of DaVinci Resolve v14.3.0.014 on Win7 Pro with 32GB of RAM, Nvidia GeForce GTX 1050 Ti.
I am sure it must be something simple. Any suggestions? "Media Offline" isn't much of an error message to run with.
The drone videos are 4096x2160 with 99,997kbps bitrate, 29.970fps MP4 format, H.264.
This is a fresh install of the free version of DaVinci Resolve v14.3.0.014 on Win7 Pro with 32GB of RAM, Nvidia GeForce GTX 1050 Ti.
I am sure it must be something simple. Any suggestions? "Media Offline" isn't much of an error message to run with.