Add second router

[Lars_Hanson]

In preparation for adding a second router to my Fios system, I read this:  https://community.verizon.com/t5/Home-Internet/Using-your-own-router-with-Verizon-Fios/td-p/1483668

A FAQ at the end of the article advises against simply plugging a second router (LAN to WAN) into the Verizon router because that would create a "double NAT".

However, in the section, "Steps for users with Fios TV", just before the FAQs, the article says to "connect an ethernet cable from the LAN port of your own router to the WAN port of your old Verizon router" in order to restore Fios functionality.

So if it creates a double NAT to put a second router behind the Verizon router (per the FAQ), why doesn't it also create a double NAT to put the Verizon router behind the second router (as is done in the article)?

Thanks!