It’s nice that you made progress !
I guess you could use wireshark to detect the ports that are used. But it could be that the port is not always the same. It may be simpler to disable the firewall on the local network, or to allow anything from the printer IP.
I know this is not much help, but it’s the best advice I can provide.
From [1] it looks like iscan-network-nt + sane-epson2 should be the solution, but we only package iscan-network-nt as part of epkowa. It may be a good idea to see if we can get iscan-network-nt + sane-epson2 working, but that would require more tinkering.