I currently have a google nest wifi system with my 4g router and I'm soon to get a starlink internet router also.
My question is it possible to have 2 routers both providing connection to the internet (one dominate and other is backup).
Is it as simple as just connecting another lan cable into the google nest wifi unit?
No, this is not something the Google / Nest WiFi systems can do on their own. You may be able to find a third party router that can do this, but it will not be inexpensive. It would need to be connected to both of your internet services and then Google / Nest WiFi would be connected to it.