Unlocking the TM20K the first time can be a little tricky. To unlock it, one has to hold down the mode selection button while sliding the toggle switch from the lock position to 1 or 2. Please take a look at the attached image to see if these steps work for you. The light will blink once when it is unlocked successfully.
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You can also check a short video here (jump to 0:42 for a quick walkthrough)
https://www.youtube.com/watch?v=932vD9rH-NY&t=114s
