How to enable RBR50 to connect with Nest Thermostat app

[gshepard]

I installed a 3rd generation Nest Thermostat that is connected to my Orbi RBR50 router and the thermostat appears as a "connected device".  (My router connects to the internet via a Verizon FIOS router, which does not broadcast WiFi.)  After multiple attempts including resetting the Nest Thermostat, the iPhone Nest app fails to Finalize its connection to the Nest Thermostat.  Any suggestions including settings that could successful enable the Orbi router to connect with the Nest Thermostat app?  Thank you!

[CrimpOn]

Many (many) years ago I installed the very first Nest thermostat.  This may have even been before installing the RBR50, which was in Fall, 2016.  It is connected to my RBR50/RBS50 Orbi system.  Thus far, I have rejected Google's attempts to get me to change from the original Nest login to a Google login.  (If it ain't not broke, why fix it???)

When the Attached Devices display shows the thermostat as being connected with an IP address, the issue is not likely to be the WiFi router.  Is the Apple device (iPhone?) connected to the same WiFi system as the thermostat?  (i.e. primary WiFi network - not the Guest WiFi?)