The app is tied to the purchasing account. If you use the same Google Play account, you can install the app on other Android devices through Google Play → Menu → My apps & games → Library → Dog Monitor and install the app from there.
If you use different Google Play accounts on all devices, you can install the app for free through Google Play Family Library. Here is a guide on how to do that: https://support.google.com/googleplay/answer/7007852