Having had many problems with the Unity Remote, I suspect you are missing the same critical requirement that I had.
As mentioned, and for the point of clarity, there are a few initial requirements you need to check to get Unity Remote working:
- You must have the respective USB drivers installed, corresponding with the manufacturer of your phone. You will often find these on the manufacturers website, and sometimes, the phone has an installer available to install the drivers when you first connect to your computer. In most cases, you should need these drivers before you can even access your phone, through file manager.
- You must have the Android SDK installed. If you are unsure, you can simply install Android Studio; it comes with it.
- You must have developer mode enabled on your android device, with at least "USB Debugging" enabled. You may have to enable more features for greater compatibility, or further debugging capability. If Unity Remote did work, and all of a sudden stops, double check that developer mode is still enabled. As I have found, it can turn it self off after a set period of time.
Once you have ticked these boxes, you may find that it still doesn't work. This is where I got stuck for some time - there is something else that Unity fails to tell you.
In most cases, you must have Unity Remote connected and running before you open Unity. That said, I find that this is an "initial" requirement. If you should accidentally close (or crash) Unity Remote during the same session, you generally do not have to close Unity again, to ensure that it opens before Unity.