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1: you need a hub or a switch with debugging/monitor port to see traffic
from all ports. Connect your PC and your device to a hub or one of those
switches (correctly configured) and you will see what happens in your
network. If you need help, post a capture file.
2: trying different WANs does not make sense on lwIP. If you have a
problem, try putting breakpoints and analyzing what you see. You should
be able to follow the function calls and observe return codes. If you
need help, post your piece of code and your result codes. you can also
enable debug output and trace what lwIP does.