I’m having some trouble mapping to an NFS share on my QNAP using my Raspberry Pi 4B with OSMC. I had no problem accessing the same file share with the same credentials on my old Raspberry Pi 3 with OSMC.
/etc/auto.nfs.shares
Content (pay attention to the colon):
/mnt/192.168.74.96/nfs 192.168.74.96:/nfs
sudo reboot your rpi to get the nfs share mounted
Since the user osmc will not be a real user on your QNAP, you should also pay attention whether the right squash option is set on your NAS: Mounting NFS - QNAPedia. You say you don’t have any issues accessing the NFS share using your rpi 3, so there there might nothing to do here.
If this is all done, you can use the Root Filesystem method to specifiy the access to the data on the NFS share. Use the path /mnt/192.168.74.96/nfs accordingly.