I use a set of three USR 56k external serial modems as a backup to regular data networks.
I've got a bunch of equipment located in remote sites around rural SW Ontario with less than rock-solid network infrastructure. The nature of the equipment requires that it's located in rural areas and often the only network available is wireless* (some sites even have to rely on 3G modems).
Needless to say, this is not ideal. But since PTSN is pretty damned reliable and goes everywhere, we just stuck modems at each major site. While we can't move much data through them, they're enough to get basic plain-text results if all else fails. Also, the network equipment is on switched PDUs that we can control from the computers, so we can just dial in to power cycle if needed.
* Strangely enough, one of the sites basically in the middle of nowhere has a fibre-to-the-basement link. Dunno how that happened, but not complaining.
If all else fails, use fire.