Contact sensor goes offline

I installed a Gen 1 system on our new construction home About 4 months ago. The base station is ceiling mounted in a hall. I have one range extender installed and have played around with its location trying to rectify continuous sensors going offline. Currently the extender is about 50 feet from the base station, separated by two walls. I’ve never had any issues with the extender going offline no matter where it has been positioned. I have two contact sensors that give me the most problems going offline randomly and coming back on randomly. One of the sensors (main door) has been an issue longer than the other (window). I changed the battery in the door’s sensor maybe two months ago and that seemed to fix the issue for a little while, but according to the app the battery was sufficiently charged and I also checked the old battery with a meter and it was still good. So I guess I’m looking for what the issue might be and how to resolve it. How long do batteries last in gen 1 contact sensors?