Some people can occasionally get the DVD player to work by doing a factory reset on the Fire TV but my understanding is it doesn't last and disappears again. Most people will just load movies on a flash drive and plug it into the USB port.
It would be interesting to find out if sideloading apps will work. My current favorite sideloaded app is called HDO.Yeah, I'll go for flash drive.
I don't use DVD a lot. listen to CD mostly.
But flash drive will do the same job.