When connected to a VPN routes are sometimes missing because the admin a the server side has missed to configure correct. So you has to set a route for the remote LAN after the connection is established. But it is not possible to add a route via a device (e.g. tun0) in the networkmanager. (e.g. ip route add 10.0.0.0/8 via dev tun0). You CAN use your VPN IP-Addr. But this may change every time you reconnect. So it were a better way to create a route via the device.
This is something NetworkManager needs to support, there is nothing we can do about this in plasma-nm. Report your request to NM bugzilla please.
Bulk transfer as requested in T17796