I have a seed node at: 54.85.252.77:39705 and will run this on the live network too.
You can specify the dedicated port with --p2p-endpoint, for me I launch with the following, but of course your other arguments may vary
./graphene/programs/witness_node/witness_node -d oct5 --genesis-json oct5-genesis.json --rpc-endpoint "127.0.0.1:8090" -s "104.236.144.84:1776" --p2p-endpoint 0.0.0.0:39705