I've tried connecting with friends over WiFi but have never successfully invited anyone over or visited another town. I keep getting error code 028-0512 or 018-0512 before connecting to the internet or before the train arrives to take me to another town. People that want to enter are also given the error code 018-0511. I've tried the manual IP Address solution, but the problem persisted.
The WiFi (D-Link wireless router) works fine for Friend Code adding, updates, and SpotPass. I don't know if this matters or not, but I'm trying to connect using a US 3DS over Chinese WiFi. The error code suggestion says that: "This is often caused by network firewalls blocking necessary network traffic." Is it because of the permanent firewalls in China? Or is it because my router is bad?