Using WiFi, peers connected to the same network segment are automatically discovered. No special configuration is needed.
Using 3G/CDMA, automatic discovery doesn't work. You have to configure static peers.
You should be able to transmit using 3G/CDMA with any carrier.
Unfotunately a few carriers block inbound connections. If your carrier blocks inbound connections, you won't be able to receive data over 3G/CDMA.