I've got a ZTE Axon 7 phone, and it generally works fine. I also have an Amazfit "Bip" watch, and I've successfully paired it with the phone via Bluetooth. However, when I try to add the watch as a "trusted device" via the phone "Security" settings, it doesn't work; the phone just immediately tells me that it "Can't add device" even though the watch is turned on and immediately adjacent to the phone. (The watch shows up as a choice in the "trusted device" screen.)
What is it that I need to do to get the phone to pay attention to the watch as a trusted device? I've done this before with the same watch and another Axon 7 phone, and I had no problems then.