Use a STUN server to assist with NAT traversal. Some DHT swarms can also assist with that. Or use a VPN or VPN-like network, like TOR, yggdrasil, IPFS p2p, etc

Use a STUN server to assist with NAT traversal. Some DHT swarms can also assist with that. Or use a VPN or VPN-like network, like TOR, yggdrasil, IPFS p2p, etc. Some are explicitly designed for that kind of use, and can form a mesh network with peers that are physically close. Unfortunately, for yggdrasil that means mostly Apple's Airdrop, though that should be possible with the more recent wifi NAN "neighbour area network" (wifi aware) and P2P.