Not sure what temp and humidity levels a terrarium is expected to have, but Shelly makes a good device for monitoring both - https://www.shelly.com/en/products/shop/shelly-h-and-t-white.
Or one with a screen (still smart) - https://www.shelly.com/en-us/products/shop/shelly-h-and-t-gen3-1/
We use the latter in our bedroom which triggers the power outlet to a humidifier if it gets too dry when we're sick.