By default, the IP address range on an AirPort Extreme is 10.0.1.2 to 10.0.1.200.
So, one option that you might have is to set the theromostat to 10.0.1.15.
Or, you can change the range settings to 192.168.0.1 to 192.168.0.200 on the AirPort Extreme in AirPort Utility.
Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme icon, then click Edit
Click the Network tab at the top of the window
Click Network Options at the bottom of the window
Change the setting for iPV4 DHCP Range to 192.168.0.2 to 200
Click Save, then click Update
Now, all your devices will receive IP addresses in the 192.168.0.x range. The theromostart will be at 192.168.0.15.