So I have an ASUS TM-AC1900 router that offers DDNS (Dynamic DNS).
I have a Rails app (call it some-old-app) and a domain (call it some-old-app.com) and got it working with the ASUS router and its DDNS so that when a user invoked https://www.some-old-app.com/something everything worked.
Basically, in my SpiritDomains (SpiritDomains is my ISP) control panel, I set things up so that some-old-app.com forwards requests to some-old-app.asuscomm.com.
(*.asuscomm.com is a free service provided by ASUS and it provides DDNS services for their TM-AC1900 router.)
When I modify
1) my SpiritDomains settings so that it forwards some-new-app.com to some-new-app.asuscomm.com, and
2) my router settings so that some-new-app.asuscomm.com is registered with ASUS
then when I attempt to do a www.some-new-app.com/something my browser says that www.some-old-app.com can't be found.
I cannot for the life of me figure out how to get https://www.some-new-app.com/something to work. I no longer need some-old-app.com so the router's limitation of only one DDNS service does not matter to me.