I just switched from Ubuntu 24 yesterday where i2p worked out of the box but the default firewall settings on Endeavour seem to be blocking (quote from the i2p help page) “the UDP port”. AIUI this port is randomised but restarting i2prouter doesn’t get around the firewall. I have no idea what I’m doing with the firewall settings and don’t want to turn off something important on accident. I don’t think it’s my router or ISP because I was using i2p just yesterday when I was running Ubuntu.
Welcome to the forum 
The other day I installed Debian and I had to enable the firewall myself, with Reborn OS the same thing, to name but a few that had not a firewall enabled. So there is a good chance Ubuntu did not have a firewall enabled too. Anyway maybe this wiki will give you some info about the firewall.
https://discovery.endeavouros.com/applications/firewalld/2022/03/
I whitelisted the UDP port it’s using both as a regular port and (when that didn’t work) as a source port. Whole lot of nothing sadly. I was able to turn off my firewall for specifically my home network but that’s really not ideal. I2p works when I do that so there’s something I’m definitely missing.