There’s dozens (or more) ways to do VPNs. All of that is going to depend on how it’s done.
What I do is run an L2TP / IPSec tunnel server on a linux server in the “remote” network.
I use the built-in L2TP VPN client on the iPad to connect to that server via a port-forwarding configuration in the router.
Since it’s IPSec based, only IP will run in it; so certain discovery services don’t work.
Once the connection is established, the iPad is then actually logically “inside” that remote network, and I can connect to my NAS boxes using ordinary file browsing apps. The only difference is that I have to connect via IP instead of by Server Name.
I use Dynamic DNS on my Router, and have a registered Dynamic DNS hostname that points to my router. So even if the external address changes, the L2TP VPN clients can still connect.