Could it be the way I'm connecting to the server in Finder? I think I just pointed Finder to the IP address without specifying a protocol. I knew nothing about protocols when I first set up the NAS.
Now you know that your NAS supports three different file sharing protocols: SMB, AFP, & NFS. AFP (Apple File Sharing Protocol) is something that Apple is abandoning and have moved to using SMB by default ... expect for Time Machine (TM) backups to a Time Capsule (TC) which still requires AFP. The TC also only support SMB v1 ... an outdated and insecure protocol. Since you are using your NAS for TM, AFP is (in my opinion) is only there as a backup for SMB. I've done data captures during TM backups to my Synology NAS and only SMB is being used.
In my case, I do not actually configure my Mac to mount the NAS at bootup. Instead, I let TM mount it "automatically" whenever it runs a backup. However, if you want to use your NAS as a file server (which makes total sense), you could create an AppleScript to mount any or all of its volumes, and then, include this script as part of your login items.
An example script follows:
Where, the NAS is called: DS916, and the share volume is called: Data. The share user name is: Administrator, and password is: mysimplepassword
try
mount volume "smb://DS916.local/Data" as user name "Administrator" with password "mysimplepassword"
end try
