Use a digital camera or the camera in a mobile phone to check that the STTL button on the remote is actually working.
Turn on the camera or open the camera app in the phone and point the end of the remote that is normally pointed towards the TV, towards the camera lens so that you can see it in the LCD screen of the camera or phone.
Press the Power button on the remote and there should be a light at the end of the remote when seen in the LCD screen. As you know that the Power button is working, this is just to prove that you're doing it correctly.
Next press the STTL button and check that the light turns on.
If not it may be a faulty or dirty STTL button contact in the remote control