Asterisk Router Behind Nat And Remote FXO Port Behind Nat

I have 2 Cisco Routers. One Local and one at Remote Location.

Both routers have 2 FXS and 2FXO ports.

Both sites can communicate with one another by using public IP of each site.

I now want to introduce Astersik PBX at local site that would cater for local as well as remote site.

I direct everything that comes to FXO or FXS port on local router to Astersik Server and world is wonderful.

However, I do not quite understand how to access the remote location with 2 FXS and 2FXO port with Asterisk Server.

I have spent quite some time and am lost.

I seek your help.