VPN is the safer way to go, but of course you still need to connect them to the internet. You just don't forward any ports (which is a good idea anyway).
If you know how to set up a VPN connection, I'd recommend this method. (Maybe your modem/router supports VPN or use a Raspberry PI to set up OpenVPN).
Don't know why your IVS lines disappear, they should be saved and stay visible. Works like that on all my Dahua cams.
Did you check whether the cam generates a snapshot when somebody crosses the (invisible) line?