Sparsebundle won't mount Mac OS Big Sur 11.2.1

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.
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