Whatever I tried I could not make the Plex app (ver. 8.12) installed on my Amazon Firestick TV connect to my Synology Ds218+ running the Plex server: it did not pass the sign-in step. App prompted the four digits code but although I typed the code in the browser at http://plex.tv/link the app did not linked. Plex server, on the other hand, did recognized that there’s an app linked.
Solved by installing version 6.12 of the apk for armeabi-v7a.
Hope I have helped someone.