I don’t have any links, but I believe somewhere there were talk of using short wave radio to broadcast the TX’s I know it was done on burst coin block chain. It’s on the block chain somewhere, but it’s been rebranded to Signum network signa is the coin.  

Burst-coin shortwave transaction

The concept of "Burst-coin shortwave transaction" involves using shortwave radio frequencies to facilitate transactions in Burstcoin, a decentralized cryptocurrency. This approach aims to enhance the resilience and censorship resistance of cryptocurrency networks by bypassing traditional internet service providers (ISPs), which may be subject to censorship or disruptions, especially in politically oppressive or disaster-stricken regions.

By utilizing shortwave radio, Burstcoin transactions can be broadcast without relying on centralized internet infrastructure, thereby increasing privacy and accessibility. This method is particularly advantageous in areas with unreliable internet connectivity or where governments impose strict controls on digital communications. The integration of radio-based transaction methods aligns with broader trends in the cryptocurrency space, exploring alternative communication technologies to ensure robust and secure transaction capabilities.

https://www.coincenter.org/decentralizing-bitcoins-last-mile-with-mobile-mesh-networks/

https://blockonomi.com/bitcoin-transactions-radio/