Mailbox sensor will not connect to bridge

I received my ring mailbox sensor today. I went to the menu item to add a device. I set the switch to external antenna and put in the batteries. Stood near the bridge. The mailbox sensor installed an update. Then I added it to a group. It then says "Connecting your Mailbox Sensor to your Bridge..." for a while and then fails. It says "Your Mailbox Sensor could not connect to your Bridge." The bridge is working okay. The bridge is online. I can control lights using the bridge. I've tried over and over to connect the mailbox sensor. No success. I've power cycled the bridge several times. I've removed the batteries and re-added the mailbox sensor several times. I've poked the reset button on the mailbox sensor. Nothing seems to help.