That’s by design and has been called out several times. Plex „optimistically“ assumes remote access is working while it’s performing a status check. This can result in you getting to see a green status indicator that’s turning red after a few seconds (up to 1-2 minutes in extreme cases).
In such a case… consider your remote access setup to not be working.
Not sure what trouble you’re experiencing. Installing an APK implies you’re on an Android system… so this is either about installing/updating PMS on a NVIDIA SHIELD (which should be working fine through the Play Store app) or you’re trying to install beta versions of a player on a mobile Android or Android TV device (where installations should also work through the Play store or the regular OS means).
Troubleshooting remote access issues can be very tricky as there’s many „moving parts“, some of them completely outside your control and most of them entirely outside what Plex can see (beyond pointing out you’re in a double NAT setup).
Did you already have a look at the related troubleshooting support article?
There’s also a guide with some more details on how to troubleshoot remote access issues. The findings/output of the individual steps will be key for any additional support/assistance.
