After correctly configuring a new wireless enabled thermostat to control the temperature of the company's meeting room, Joe, a network administrator determines that the thermostat is not connecting…

A captive portal requires a user to open a browser and authenticate or accept terms before internet access is granted. IoT devices like thermostats lack a browser interface and cannot complete this authentication step, so they remain blocked from internet access even though they are associated with the AP and have valid IP parameters. This also explains why human-operated mobile devices on the same network work fine - they can interact with the captive portal - and why the thermostat worked at Joe's residence, which presumably has no captive portal.

An incorrect encryption algorithm would prevent the thermostat from successfully associating with the AP entirely, but the question states the thermostat is already associated with the AP.

An incorrect WPA2 shared key would also cause the device to fail during the association/authentication phase with the AP, not after a successful connection has already been established.

If the DHCP scope were full, the thermostat would not receive the expected network parameters (IP address, gateway, DNS), but the question explicitly states that the thermostat did receive the expected network parameters.