Time Machine backup on NAS device

My environment is a MBP-M1 with Big Sur 11.1, 16 GB memory and 2TB storage. I have TM backing up on a WD EX2 Ultra and this seems to work ok so far.

If I stop TM and mount the TM sparsebundle on the NAS, I can see and examine all the backups in Finder, as displayed:

BUT the following problems arise:

1. I cannot copy any of the backups: even Cmd-C fails with a beep.
2. If I open a zsh window and cd to the /Volumes mount point of the disk image, the ls command does not show ANY of the backups:

It is of course very important for me to be able to manipulate the backups in a shell. Can anyone shed light on this? Thanks in advance.