Unable to access RND4210

The firmware for your NAS has not been updated in years, so I doubt you missed one.  And there is no reason to believe that would cause you to lose access even if you did.  When did you lose the ability to connect, and did you lose it with the GUI or SMB, or both?  There are two issues with your older NAS that can keep you from accessing those:  It is limited to SMB V1 and TLS 1.0.  There are work-arounds for both.  But lets first see if the hardware is actually OK.

What do the display and RAIDar say about the NAS?  If RAIDar doesn't find it, then remove the drives (labeling them so you put them back in the right places) and see if it then finds it and both the display and RAIDar report "No disks".  Also, what OS version does RAIDar report if it does see the NAS? 

From what you find out there, we'll move on to what you might need to do to access or recover your files.

I'm not surprised Geek Squad was of no help, they are clueless when it comes to Linux/NAS (and many are basically clueless on everything, they just follow "cookbooks").  That they first suggested you find another chassis without even considering that the problem might be the OS is an indication of their level of cluelessness.