I recently got a braven Ready Prime version and had the exact same problems was fully charged and would turn on even if I held the on button for 10 or more seconds and the reset button wasn’t doing anything.
i then put my portable charger (might work with a phone or anything that can be charged) on charge through the speakers USB port and the speaker was charging my portable charger I then left it for a couple hours and my portable charger was fully charged, then I put the speaker on charge and suddenly all the lights came back and it turned on straight away, so you may have to drain the speakers battery and charge it again for it to turn back on, hope this helps anyone