Thanks for reaching out here in the community.
I'd like to check a couple of things to further isolate the issue. Are they connected to the same Wi-Fi network? You could try to restart your Wi-Fi router including your phone and thermostat. Were there any changes made?
Let us know how it goes by replying to this thread.
Since this thread hasn't had activity in a while, we're going to close it to keep content fresh. Feel free to submit another post, and provide as many details as possible so that others can lend a hand.