This issue remains unresolved, even after replacing our 2016 Nest Learning Thermostat with a brand-new model purchased directly from Google. Besides sometimes being found "offline" the thermostat does not respond promptly to app control -- if the app is used to change the temperature setting, response may take several minutes instead of only a couple of seconds. It seems as though there is a breakdown between the app, the Google server (if that is in the chain), and the thermostat. This loss of functionality may have begun around the time we switched from cable internet to fiber, though no changes were made to our in-home router or network. I've reviewed router settings repeatedly and yes, old thermostat was removed from my account, router restarted and IP V.4/IP V.6 dual-stack is running. Still at a loss as to what has gone wrong and how to correct this.
