VPN in a 2 Router System

There appear to be three options:

• The Fritzbox supports VPN. (See page 120 of the user manual:
  https://images-eu.ssl-images-amazon.com/images/I/C1Imnph2H8S.pdf 
  This would require the Orbi to be placed in Access Point (AP) mode, which is found on the Orbi web interface Advanced Tab, Advanced Setup (the bottom menu item).
• The Fritzbox also supports port forwarding for one device on the network:
  See page 115 of the user manual.
  Orbi VPN uses port 12973 for the TUN version of OpenVPN, which is the most common version
• Have the Internet Service Provider (ISP) replace the Fritzbox with a "plain modem" which has no router functionality.  Spectrum (in the US) offered me a choice of either (a) a simple DOCSIS 3.1 modem, or (b) a combination modem/router/WiFi (like the Fritzbox) and I chose the plain modem.

Two other methods for dealing with a "Double NAT" (do an internet search) do not appear to be possible with the Fritzbox:

• Putting the Fritzbox into :"Bridge" or "Passthrough" Mode, where it acts like a simple modem and has no router functionality.
  I cannot find the term "Bridge" or "Passthrough" in the user manual. 
• Defining the Orbi router in the Fritzbox DMZ, so that all data packets bypass the Fritzbox router functions and go directly to the Orbi router (in addition to passing through the Fritzbox router).  I cannot find "DMZ" in the user manual, either.

This is not a trivial undertaking.  Good Luck!