3rd party repositories has been closed?

Yes Entware-ng can run on a NSA310, provided you exchange libc by a patched one. Unfortunately that's not just any libc.
I couldn't find a backup of my patched libraries.

@waiting4 and @MGD, Entware-ng will run fine on a 326, but it's not a step-up to Plex. Plex is not available as package for Entware-ng (nor for FFP). The way to install Plex is to download a package for the Synology, extract that, put it in the right directories, create some environment variables and start the binary. @barmaley2 has researched this, and I have put his findings in both an FFP package and a zypkg package, but in both cases it's basically a script, and you'll have to provide the compatible Plex package.
If nas-central.org is back, both scripts are available.

For the NSA310 there is a complicating factor. Plex dropped support for Armv5 a few years ago, and I don't know if there are still Armv5 packages for download, nor if they are still compatible with clients.

AFAIK a NAS326 is powerful enough to run Plex, although I don't have experience. It won't transcode, but it won't transcode on any Arm NAS.
1300MHz Armv7 is not really comparable to 1200MHz Armv5.

OK all clear. I thank both and I hope nas-central will be back online soon.