I can confirm that the two ethernet ports on an AR300M can indeed be configured as two wan ports. You need the mwan3 package to manage the ports giving load balancing on a connection basis and failover in the event of one of the wan feeds going down.
This also gives the advantage of doubling the throughput over that of a single 100Mb/s ethernet port even if connected to just one ISP’s router with the two ports.
We even sell a captive portal device based on the AR300M with dual wan functionality built in and switch-able in the UI.