Not the main focus of your question…
You will still be able to use Plex with the apps currently installed on your iOS / TVs – just not with future updates of those. So you might need to disable automatic updates for those apps (unless you find what you find that “sweet spot” between 1.15.2 and the version breaking things for you on that RN214).
Sorry to say I’m not familiar with those Netgear NAS, so I cannot be of much help as for the issues that model gave you.
Edit:
adding the announcement / mail you’re referring to in case other users had missed it
