Sparsebundle won't mount Mac OS Big Sur 11.2.1

Hi,
I’ve got a MyCloud EX2 Ultra running old firmware (2.41.116). I’m trying to access a sparse bundle from a new Mac Pro with Apple silicon, running Big Sur 11.2.1.
We set it up so our photo library was on the sparse bundle. I can no longer mount the sparse bundle (and so can’t access photos).
I was getting a dialog that said “no mountable file systems”. I rebooted the MyCloud and now get a dialog box that says “Resource Temporarily Unavailable”. After posting this initially, I tried again and am now back to the “no mountable file systems” message.
How can I mount my photo library?
Do I need to update the firmware on the MyCloud EX2 Ultra? Update to MyCloud OS 5? Is it an issue with system compatibility?
Any advice will be appreciated.
Thanks,
Michael
p.s. I can mount the library on a different machine, this one an older Mac Book Pro running Mac OS Catalina, 10.15.7

Hello,

We would recommend you update your My Cloud EX2 Ultra to the latest OS5 version.

Hi Logan.S,
Thanks for the response. FYI, I spoke to a member of the WD Support Team by phone last week, per your suggestion. I was told by the rep, after they consulted with a colleague, that actually this is an issue with Apple and that I should contact them, and, after asking if I should upgrade to OS5, I was told that upgrading to the OS5 version, as you’ve suggested, would not solve the problem.

I wasn’t happy to hear that I should reach out to Apple, though in a way it makes sense as I’ve seen posts that say with the introduction of Big Sur, NAS set-ups such as mine would not work anymore as a repository for a photo library.
So, I know have somewhat conflicting advice from WD support. Which should I follow?