Before I go into the details, the problem is actually as specific as the title suggests. I suspect the issue is with DD-WRT, but I'm not sure what it is.
So I got a new router (Asus RT-AC68U) and flashed it with DD-WRT. I have both the new and old version of the chromecast, and an old version connected via wired and wireless adapters. My Plex server is running on a Netgear ReadyNAS app.
* Play Plex shows on my android phone app screen via LAN
* Play Plex shows on my desktop via the web browser (chrome)
* Play Plex shows on my chromecasts via casting from the desktop in chrome browser
* Play Netflix on my chromecasts via my android phone
* Pause, play, and change volume of an already playing Plex show on my chromecasts (initiated on the browser) via the android app
* Initiate play of Plex shows on my chromecasts via casting via the android phone app (the error is a small popup that says "Failed to play on
I have tried:
* Resetting the Plex app data and cache
* Rebooting the router
* Enabling UPnP on the router (haven't rebooted yet, but plan to)
* Assigning static IPs to the various devices invovled
To be clear, none of these problems happened on my old router, but I did not test the new router's stock firmware before flashing. I have no reason to suspect there's a problem with the chromecast or NAS server. The problem appears to be specifically limited to the android app and chromecast with DD-WRT firmware as an intermediary.
Would love some suggestions on stuff to try!