Hello Flavio,
You need to be careful when exposing your system to the internet.
You might be inviting unauthorized access... 
You should consider creating a VPN with your remote locations or end users, and provide the access through the VPN only.
Anyway, it would depend on what do you have facing the internet (a xDSL modem? a firewall? a router?).
I believe that the most easy way would be to create a NAT (or "DNAT") rule in such a way that whenever your internet public IP is accessed at the SAP port (32$$, where "$$" is the SAP instance number) it forwards the request to your SAP (internal) IP address.
Using a saprouter would not be mandatory.
Regarding your public IP being dynamic, you would have to find a way to inform the users to update the IP address at their SAP GUI every time your IP changes, or you can use a free dynamic DNS service.