AX12 router does not route when a different subnet is specified

When I do a standard setup all the devices connect to the internet without a problem.  But if I change the LAN settings, setting the LAN address for the router to a different subnet (172), no devices (wired or wireless) can get to the Internet.  I can connect to the AX portal, and the AX can access the internet (check for firmware and speed test), but the devices (laptops, desktops, tablets, etc) on the 172 subnet can access the internet.

Any thoughts?  I paid for a tech support call, and the person had absolutely no knoweldge past the KB tool she was using.