WiFi looks possible. Did you try it?
About the offline. They always go offline. I can't reveal all I know about this but let's say I do design work on mobile dvrs and have been overseas to correct product issues.
Parting tip: Put it on a timer to reboot weekly or so.
I am installing 2 DVR systems, stand alone linux based, in two adjacent buildings, maybe a 100 ft apart with clear line of sight. The dsl modem will be in center of one building and I can run an antenna. There will be no other devices on the network and DVR's will be in a locked room. Issues would be no one local to perform any maintenance and cost. I don't want to have to be called repeatedly because it went offline. It is possible to bury a cable but very difficult. Any wireless Ideas?