If you don’t want to get NFS no matter what, but you’d just like to get a FSTAB mount working quickly, it might be good to switch to SMB FSTAB mounting at this point. With SMB you can use authentication and it might still give good enough throughput (but getting rid of the hassle with NFS, if this isn’t solvable in a timely manner, which it doesn’t seem to be):
And I thought, my NAS (WD MyBook Live) was a difficult setup with NFS… It doesn’t have any GUI configuration options for NFS, everything had to be done via SSH. But it looks like it was a lot less complicated than Synology seems to make the whole setup 