I have FTTH (fiber to the house), the ONT is inside the house and the ISP supplied router is connected to it. I'm considering getting a Nest router however due the fact that the ISP supplied router provides a VOIP phone line I wont be replacing the ISP router. My plan is to connect the nest router to the ISP router and turn off WIFI on it (the ISP router). My question is can the Nest router be configured in an access point mode?
Also I'm aware you can add Google WIFI points to the Nest system, I'm also aware if you had an all WIFI solution you can create a wired backhaul by connecting them all back to a switch. Could you create the same wired backhaul setup with 1 Nest router and a few WIFI points?
The Nest WiFi Router unit can be configured in "bridge" mode. However, in this mode it will not support adding secondary units to expand the coverage area (you would also lose a number of other features, including Guest WiFi, prioritization, and more). Running with double NAT can be an issue in some cases, but doesn't cause problems for most people.
Wired backhaul of Google WiFi units (or additional Nest WiFi Router units) works quite well, but they all have to be connected to the "inner" network created by the primary Nest WiFi Router (or Google WiFi) that is connected to your internet service. Using an inexpensive unmanaged Ethernet switch to build a network to support that from the LAN port on the primary is recommended. Note that wired secondaries are not supported if the primary is in bridge mode – the wireless mesh connection is still running as a backup and managing all of them as a group requires the primary to be in router mode. I'm a little unclear on what you're asking in your closing question, but I hope the above answers it. If not, please don't hesitate to ask for clarification here!