Why beacons and not a GPS/geofence system? Is there some advantage to using beacons I'm not familiar with?
A geofence is just a shape on a map, which is a latitude x longitude grid. If a device knows A) the shape of the geofence, and B) its own location, it will accurately determine whether it's within a geofence.
Neither of those things (A or B) require an internet connection. GPS operates independently from the internet. It's a one-way transmission (satellites -> device).
It's possible to detect location more quickly or more accurately using other sources to augment GPS. These include WiFi SSIDs, as mentioned above. That still doesn't require an internet connection -- only a WiFi adapter.
It might be a good idea to look into post-install "city pack" downloads, where you can download some subset of data for cities you care about (or are about to travel to). Might be kind of neat also, and opens up a cognitive gate to letting people potentially just "browse" beacon information remotely in the cities they care about.