Qnap SS 439 Pro | Custom OS Upgrade

I have a Qnap SS 439 Pro with 2gb of RAM (Originally it had only 1gb). I want to run Plex or something similar, but the EOL OS QTS Version is 4.2.6, so the OS is pretty old and does not support many Apps anymore. I was allowed to install an old version of Plex, but it does not allow to optimize videos, and my wifi is too bad to stream the whole thing to the devices it should reach. It is also not possible to connect the Plex Server to an Account, so it is not found by the Plex client server. It is possible to find and play the files with the VLC player app, but the cast to TV/Chromecast is to taxing on the wifi.

Is there a way to either update software/QTS os or are there custom ROMs/Distros for NAS OS available, like I would update old phones with a custom ROM?

Thankyou in advance.