@StevenP Solved. Turned off fast roaming, beamforming and 5 ghz. Then reset thermostat. Connected. Left fast roaming off as it might still create issues but turned 5 ghz and beamforming back on.

Nest Thermostat (Self Learning model) will not connect. Thermostat sees network but won't connect. This began with the installation of the Deco router. It worked perfectly with previous router setup...