How does select regions?

Several users in Central Asia are getting routed to ap-southeast-1 instead of the European regions. This is a poor user experience. Central Asia in general has much better connections to Europe through Russia (10-20Mbps, 100ms latency), than to China or India (500kbps-1Mbps, 200-400ms latency). How does select which region is used? Is there a way to select a different one? (Maybe by using a different DNS server?)