I had missed that you’re on a pretty old version of Plex – both on your server and client.
Plex for Android/FireTV 8.2.0 is addressing an issue where the filters don’t show when you’re on an old PMS version.
If possible you should however upgrade your server.
The latest public release is version 1.19.4. The version you have installed is from November 2017.
PS: Even that Plex client is dating back to November 2017 (current version is 8.2.0)
