Amazfit T-Rex Pro not turning on if unplugged from the charger

i've had this watch for 3-4 month although i got it second hand ,it was working just fine and then yesterday it turned off i plugged it in the charger and it said the battery was 70+% and a soon i took the charger off it turned off again , ive tried letting it charge overnight as well as leaving it for a while with no charger, i did reboot it and nothing happened ,i never had issues with battery before i dont know how it suddenly stopped working , the battery is full but if i unplug the charger it turns off and i cant access the screen while its charging to do factory rest , does anyone have any idea what could it be and how to fix it ? thx