[beepbeep]

Data security is really not a issue, even if you carry storage on board, as most NAS/SAN storage racks offer strong on-the-fly data encryption. So even if plane crashes, nobody would be able to restore the data without correct key.

But if you really insist on data link through radio, your best bet is probably ad-hoc LTE (=4G) network. It's made with moving radio datagram transmission in mind and has all the stuff to cope with it. (But due to time+frequency division multiplex scheme, I believe there is a hard limit of 100 km regardless of signal power due to timing issues.)

Basically, you would need a portable LTE cellphone station(s) for each cell within 100km of the plane. Such stations usually need fibre backhaul in-between but I suppose something could be cobbled together with microwave links.