Mongodb replica set across AWS regions or subnets or data centers

I seem to have run into a blocking problem. I want a certain collection to be replicated across multiple AWS regions/subnets/data centers (eg a global lookup table or a global user credentials table) i.e. all data replicated across all regions and it can be updated by application layer of any region. In order to do that do I need to provide public IP address and therefore put all my mongodb EC2 nodes in public network so that replication can be done across the regions and in case of writes from application, a primary node in a different region / subnet can be accessed by my application layer? I don't see how else replication will happen.

Thanks and Regards, Archanaa Panda