Have you tried setting up an NFS share from the Ubuntu system? It seems
like you've mentioned SFTP, UPnP, but it seems like NFS is the more
appropriate protocol here.
I have only used the Pi with a Windows SMB share but I imagine the
process isn't terribly different on the Pi side of things (you just select
NFS share and put in the details). Here are resources I found specific to
Sidenote: this question probably shouldn't fall within StackOverflow.
It's not programming related. Maybe Superuser or Ask Ubuntu.