Configure wr21 as wireless gateway

I want to repurpose an existing wr21 to become an internet gateway by connecting my wifi router to an Ethernet port on the Wr21 and then accessing the Internet over 3G. I already have a working cellular connection. Want to figure out if this is possible? Settings or scripts I might need? I think this must be a very easy application, but cannot see it specifically explained anywhere.

Yes it is possible and it is sort of explained on the Digi website in the documentation section of WR21 support page:

AN041: Ethernet WAN Failover to Cellular/Mobile

The trouble is that if you are using a residential wifi router you may not be able to adjust the settings in the device as needed because you would need two subnets with the router operating in one and the wifi operating in the other which might not be possible in a residential router.

Thank you Nicholas!! I will be sure to address the issue of the 2nd subnet.