Seems my Belkin N750DB router is knocking off my new Nest (thermostat).
Both communicating with each other fine since the Nest's install on Monday (this is Thurs evening).
I have added the Nest's MAC address in the Router's Mac add filtering section. But can't think of any thing else to check.
Also noticed the date & time on the (router's) setting screen is all wrong. It shows the date as Jan 1, 1970 and the time is 1:11 am (its 9:07pm now). I don't see how to correct the time & date though.
Turns out by changing the primary & secondary NTP servers corrected the date & time, which then allowed the Nest to see the router.
I have had this router 2 plus years and this is first time after a new year that that date & time not correct.