Door sensor offline and can't reconnect

Hi, I have 6 Ring sensors, 2 are now offline and this is impossible to reconnect, they stay offline. I changed the battery but t=same thing and the battery is full charge. Both 2 sensors seem out of service after 4 months, disappointed. What I can do and how to know if they are ouit of service and how to try to reconnect and how to replace for free ?