Network devices provide seamless offloading of data communications from a service provider's cellular network to an alternate access network outside of the service provider's cellular network. After a cellular mobile device has initially been authenticated by devices in the cellular network to communicate with a packet data network via the cellular network, the cellular mobile device is configured to leverage its prior authentication on the cellular network and automatically obtain credentials for use in logging onto the alternate access network, without requiring any action by a user of the cellular mobile device. After the cellular mobile device is logged on to the alternate access network, the cellular mobile device sends wireless data communications to the packet data network via the alternate access network instead of the cellular network.