I'm not with Laminar Research, just a fellow simmer in the UK :-)
My understanding is that the 'demo' version of X-Plane 11 is the same as the full digital download version, although there may be a time limit in terms of flying time in one session (15 mins?)
In any event, you are correct, if you are using any downloaded version, then you will need to be connected to the internet at all times, as LR makes periodic checks on the validity of your X-Plane 11 copy and tracks the necessity for updates etc.
There may be another option though, given that you have no internet connection and that is to purchase X-Plane 11 on DVD's. Please see following link:- https://www.x-plane.com/product/x-plane-11-dvd-set/
The only problem with this option though, if you still have no internet connection, is that you won't be able to receive updates going forward.
Hope this helps