Here’s the case. I always play over WiFi connection, but sometimes, the system persistently matches me against overseas players.
To play with domestic players, I temporarily connect my device to cellular data, relaunch the app, then join a game. I will reconnect it to WiFi after selecting a hero.
Tip: I know that I’ll be matched against overseas players if my ping is over 20. That’s after selecting a mode. If that’s the case, I do the workaround.