For NowTV Entertainment, we only use 2 devices to watch; an iPad and an Amazon FireStick.
However, we've recently had an error message pop up when watching on the FireStick saying that we're active on too many devices. We then went back to the iPad and now we're getting the same message. We're unable to add or remove devices from our account, so we are unable to watch anything on NowTV despite months of no issue.
I don't have a Firestick, but does it give you a back out option which ends with "close app," or similar?
For the iPad, make sure you stop the stream and go right back to the home page, then double press the iPad button and swipe up on the app icon to stop it being active. Or swipe from the bottom and do likewise if you have a full touchscreen version.