You are talking about two completely different Static IP questions here.
If your ISP has issued a Static IP address to you for the AirPort Extreme, this is the IP Address that you must use in order for your AirPort Extreme to connect to the Internet.
Open AirPort Utility, click on the AirPort Extreme icon and then click Edit
Then click on the Internet tab a the top of the screen.
The setting for Connect Using must be set to Static, and you will need to enter in the IP address, DNS servers, etc information that your ISP has provided for you to be able to connect to the Internet. In the screenshot example below, you would enter the Static IP address that your ISP provided to you in the IPv4 box.

For now, click the Network tab at the top of the screen and insure that the setting for Router Mode is set to DHCP and NAT.
We first need to make sure that your AirPort Extreme can connect to the Internet using the Static IP address that you have been assigned.
Then, we will tell you how to set up Static or Reserved IP addressed for each of your devices on your local network if this is what you want to do. Normally, this would not be needed, but it can be done if you wish.