I have/use a Synology 1815+. I mainly use NFS shares and also iSCSI to Proxmox, VMs etc.
SSH into your Proxmox host: /usr/sbin/showmount -e ip_or_fqdn_to_your_synology_appliance
this will show you all the NFS shares available, if any
If you haven't already, on the Synology appliance, control panel, shared folder create a new share or edit an existing one. you want use the NFS permissions tab.
Pretty straightforward. If you need detailed steps I can provide those.