Hello Cfy,
Thanks for posting here in the Community. To connect your Nest Doorbell battery to your Nest speaker or display, both devices should be in the same home structure. Once both devices are set up on the same home structure, you need to turn on the visitor announcement so that the speaker will announce when someone rings the doorbell. Here is a link to get doorbell visitor announcements on a speaker or display.
Let me know if you have additional questions.
Best,
Edmond
