Are you controlling the Onn device in any way via the remote? The only way I have ever gotten that "stuck" lock is when I was playing with the device locally as well as having adbtuner control it. How does it behave if you just control it through the UI of adbtuner where you test tune the channel? Does it allocate the tuner then release it properly when it is done?
FWIW you may want to pick up another ONN device or two so that you have some other tuners available once you sort all of this out.
Also go into developer mode and enable stay awake while charging and see if that helps.