Having the pretty much the same problem.
I have and Echo 2nd Generation
Server is ARM 6.x RN2xx on Netgear RN212 NAS drive which is where my media library is stored. Plex Server points to this ok. Port 32400 is open and NAS drive has static IP, I can ping this externally so I know it is open ok. Router is Netgear Nighthawk R7000.
Remote access is enabled in web app pointing to port 32400 (Private and Public) and Remote Access is enabled on the router. Alexa Skill is enabled and authorised to my Plex account.
I only have music library set up at the moment, but I can play my music from the NAS Library using Plex App on Fire Stick, Android phone (Huawei P9) and Windows 10 laptop. This works locally and remotely.
Alexa confirms that my NAS drive is my server, but I can only ever get her to identify my player as either Plex-Web Internet Explorer or Fire TV Stick - I can use Alexa to switch between them as my primary player. But when I ask Alexa to ask Plex to play any music I get the response - “trouble accessing your Plex skill”
I can see in my devices - Fire TV Stick, Amazon Alexa, Plex-Web (Internet Explorer), Android phone and Window loptop in the list of devices.
Any help would be greatly received, being able to play all my music collection through Alexa would be fantastic. If I could get this to work then I’d be seriously interested in setting up my film library, photo library and going premium but unless I can get the music to work I am more than reluctant.
1 question that keeps bugging me, I don’t see Plex displaying an Echo 2nd generation in its supported device list (it only shows 1st generation and doesn’t say 1st or 2nd generation in its supported device documentation (it just says Echo)