Very poor wifi connection

Video Doorbell 3. I installed it yesterday. It works fine with the door open, but not with it shut. My router is almost vertically above the door, about 8 feet above. Would a WiFi extender help as I cannot move the router very far? I am using the 5GHz connection. Would it be better to use 2.4GHz and if so then how do I change it? Many thanks.