Thanks for that suggestion.

I think I need my head examined. Maybe it's because I woke up early this morning and not thinking straight.

I changed my setup (again).

Cable modem -> WRT -> RT

WRT is 172.20.0.1
RT is 192.168.15.1

Connected WRT LAN to RT WAN

On the Internet Setup page, should I have "Obtain IP automatically"? If so, should it get a 172.20.0.x address?