So I was able to fix the 018-0512 error code on my 3DS by turning on the DMZ. However, since we both use the same router for internet, we can't use the same IP address on our 3DS's or neither of us can connect to the internet and therefore, only one of us can have DMZ on our 3DS's and visit other friends without error code 018-0512. Help please?
Tbh its not due to you being on same router.. today I know 7 of us have been having same issue on and off.. its horrible, We think its due to UK launch and some sorta server overloaded. We have seen the same error message and we are not in same room or house. So that error code is something else, something with match making I think.
The same thing happened to me and my friends during the North American release. However, I WAS able to fix the problem on my own 3DS by turning on the DMZ to my 3DS's IP adress and now I'm able to play without any issues. However, as far as I know, you can only turn on the DMZ on ONE IP address. My 3DS and my sister's 3DS can't share the same IP address. As a result, only my 3DS can connect to the friends I couldn't play with before.
Also if you guys are having the same issue, try this. http://www.nintendo.com/consumer/wfc/en_na/ds/results.jsp?error_code=018-0512&system=3DS&locale=en_US
The support website finally was useful for once. Make sure you also do the last part with turning on the DMZ on your 3DS.