Through which IP address will my server be communicating if I don’t use a Floating IP address?
In case you don't use an Floating IP address, the server will be communicating with the IP address of your Router as its source address.
You can verify this by issuing the following command:
dig +short myip.opendns.com @resolver1.opendns.com
The response will show you the IP address from which your server is communicating with.