If NFS is not enabled on the NAS, you must first enable it. The checkbox for the setting can be found under:
Control Panel → File Services → Win/Mac/NFS → NFS Service → Enable NFS
For further details and screenshots on how to enable NFS see these instructions within the Synology guide on setting up NFS.
sudo apt install nfs-common
Create a target directory for your mount point, and execute the following command adjusted with the NFS server IP and the mount directory:
sudo mount 192.168.1.214:/volume1/Downloads /media/NAS/Downloads
If it is desired for the NFS share mount to persist after reboot/shutdown, add an entry to the
192.168.1.214:/volume1/Downloads /media/NAS/Downloads nfs rsize=8192,wsize=8192,timeo=14,intr
The mounted share should now be accessible on the client. Check via the CLI or file manager of your choice:
These steps are also working for Ubuntu 19.10, (and I assume they work on versions inbetween, such as the 18.04 LTS release, and also probably the 16.04 LTS release prior).