After an agonizing wait, my son's copy of Splatoon finally arrived today. It was purchased so he could play it at my parents' house while he is spending time there over the holidays. He excitedly fired it up, and was able to connect to the lobby (in which you can see other people who are online) but every time he tried to join a battle, he got the error message "Unable to connect to partner's device". This was accompanied by error code 118-0516 or 118-0502, which a quick Google suggests signifies a connection error.
We tested the Internet connection in the Wii settings - said it was fine. We cycled the router. We tried using Google's DNS settings. I followed Nintendo's instructions to assign the Wii a manual IP address, intending to add it to the router's DMZ. Then I came unstuck. My parents have a Spark issued Technicolor TGN582n on an ADSL connection. I tried following their instructions for adding a device to the DMZ, but while I could find the Wii in the main device list, when I went into "Click Assign the public IP address of a connection... under Pick a task" I couldn't find the Wii in the drop-down list of devices. I also was confused about whether I was putting the whole device into DMZ mode and putting their whole network at risk, so I chickened out.
Next I started to try some port forwarding, but while I could find the area in the modem to do it, I didn't really know what I was doing so decided to leave it. Hence we are back at square one.
From what I can find online, a lot of people have this issue, and a few have solved it using the DMZ solution. Others have suggested getting an Ethernet adapter for the Wii U, saying it fixed thei connection woes. I'm happy to get one if it is likely to work. I'm even happy to buy my parents a new, more configurable modem, if that is what's needed (their wireless runs at about 4Mb so I'm thinking a new modem might help improve that too).
Please help - I'd be very grateful if someone could assist me in sorting this out for my son before week 2 of the holidays. He's been so looking forward to playing this and is mighty disappointed at the moment.
