So.....
I tried assigning my console multiple times using Dr. Mario Online RX, but it didn't work.
I check my FC, connect to Wiimmfi using that without disconnecting, and assigned, but it didn't work.
I'm sure about 2/3 prerequisites. But the "The Wii connection and this formula request must use the same internet connection (same IPv4 address)." stuck to me.
Maybe the Dolphin used different IPv4 while bridging network. IDK.
So, yeah. Does anyone know the answer?
Added poll.
EDIT: Its not Dolphin's fault.

Coincidentally I just been activated.
So I tried to do that, and it worked...
So its not Dolphin.

@Leseratte Should this issue be expanded in the multi-game thread or something?
Since this might be a problem to people, especially to set up the console before activating MKW.