Around the turn of the century, in a network the size of Europe, we had OOB comms to the core routers via ISDN/POTS. We experimented with mobile phones in the racks as well, much to the chagrin of the old telco guys running the PoPs.
Try running a full node for ETH on a beefy desktop and you quickly run into the limits. There is also the maximimum speed of light, which will make scaling interplanetary a bit difficult on high speed block times.