Watch only works on the charger

I have an lg g watch r and I loved it i don't want to get a new smart watch because this one looks the most similar to the watches I wear. My g watch worked all the time no problem one night I went to bed and when I woke up i saw it didn't charge so I tried to put it on again. That didn't work. Then I tried getting a new cradle that still didn't work. I've tried Factory reseting it and that hasn't helped either. The watch works completely normal and fine when it's on the cradle the moment I pull it off it shuts off. (It does the powe ring down screen not just turning straight to black) any suggestions on what to do next. I'm thinking I need to change the battery but it didn't even start to slowly get worse it just went from working normal to not working at all off the charger please help and if I do need to change the battery where would I find it because I've looked and I can't seem to find the battery.