PostgresQL master-master (master-slave) replication with only 1 external IP?

Question

PostgreSQL

The task is this.
There is a DB on the server which has external IP.
Some results are stored in this database and they can be viewed through the site.
Then I go with another server to the field, where there is Internet according to the scheme.
1. The wire came into the building
2. I connect it to the router
3. Wires diverge from the router to other routers
4. The end point in the field, which ends with the wire into which I connect the switch (without the possibility of forwarding, etc.)
5. The wire goes from this switch to the server (in the field)
I.e. There is no way to do port forwarding.
The same database is spinning on this server, but results come to it and it must be spliced ​​to a database that is on the "Internet".
Whether there is an opportunity to configure such replication?
I know that MSSQL has such an opportunity, the main thing is that 1 of the servers has an IP and from the second you establish a connection to it and they replicate over this connection, is there something similar for PG?
And I want that at the moment when I was in the "field" the master was the database, which is in the "field", and when not in the field, then the master is the database on the Internet.