'The disk you inserted was not readable by this computer' when connecting external hard drives
Depending on the age of the DiskStation the HDs should either use EXT3/4 filesystem or Btrfs filesystem. Both filesystems are natively not supported in macOS (hence the "not readable" dialog). If the NAS HDs are formatted in EXT3/4 you can try the EXT filesystem package for macOS by Paragon software (trial is available).
https://www.paragon-software.com/home/extfs-mac/

If the filesystem on the NAS HDs is Btrfs, no luck with macOS. There is no Btrfs filesystem "driver" for macOS available. You can then only try to mount the drives on a system capable of reading Btrfs (another Syno NAS/Linux box etc.).