“Set a port and forward it to your i2pd instance in your router or (if available) use UPnP” https://github.com/PurpleI2P/i2pd/issues/1694 “If a static IP address is available, you need to either forward the port on your router or enable UPnP on it. i2pd supports UPnP and can open the port when the program starts.” https://github.com/PurpleI2P/i2pd/issues/1650 (machine translated, member of a PurpleI2P)

If UPnP is enabled on router and he is firewalled (I2P reports that), then IMO he should try forwarding that port traffic on the router to his LAN IP. But I have not and seen no proof that would show if firewalled and non firewalled makes any difference in I2P. In bittorrent it makes significant difference, it allows peers behind NAT to interconnect and bittorrent is using at least one technique to workaround peer connections behind NAT (but can still fail to establish connections between two passive/firewalled peers).

My assumption is that forwarding port allows peers (other I2P users) to connect you even they are behind the firewall. Two firewalled peers can not connect in a bittorrent, but I2P is not bittorrent… But when it warns you about being “firewalled” it suggests some negative impact to me and suggests there may be similar connectivity issue like with bittorrent. Ports needs to be forwarded on router device (if applicable) or DMZ(if you are able to lock down your connectivity using your computer firewall - DENY/block policy by default and whitelist only listening ports of I2P, torrent client and similar), on VPN server, and opened in your firewall (if applicable).