It has to be done using Airport Utility. I had mentioned the steps for configuring from a Windows PC. Since you are using a Mac, you can use the following steps -
Open Airport Utility. To open Airport Utility, click Launchpad, put Airport Utility in the search box. Click Airport Utility to launch it.
Go to Internet tab. Connect Using = Static. IPV4 address = 192.168.3.1; Sub net mask = 255.255.255.255; Router address = 192.168.3.1; Click Update; This will set the IP address for the Airport Express.
Change Connect Using = PPPoE; Account Name = <your ISP ac name>; Password = <your isp password; do not leave blank>; Service Name = <leave blank>; Click Update;
Click Network; Router Mode = 'DHCP Only'; DHCP Range = 192.168.3.2 to 192.168.3.200; Click Network Options...; DHCP Lease = 1 day; Save; Click Update;
Click Wireless; Click Wire less options; You can select either of 802.11n only (2.4GHz) or 5GHz, if all you wireless clients are 802.11n compatible. Use 2.4GHz for slower speed and longer range; Use 5Ghz for faster speed and shorter range.
Let me know if it works.
Open Airport Utility. To open Airport Utility, click Launchpad, put Airport Utility in the search box. Click Airport Utility to launch it.
Go to Internet tab. Connect Using = Static. IPV4 address = 192.168.3.1; Sub net mask = 255.255.255.255; Router address = 192.168.3.1; Click Update; This will set the IP address for the Airport Express.
Change Connect Using = PPPoE; Account Name = <your ISP ac name>; Password = <your isp password; do not leave blank>; Service Name = <leave blank>; Click Update;
Click Network; Router Mode = 'DHCP Only'; DHCP Range = 192.168.3.2 to 192.168.3.200; Click Network Options...; DHCP Lease = 1 day; Save; Click Update;
Click Wireless; Click Wire less options; You can select either of 802.11n only (2.4GHz) or 5GHz, if all you wireless clients are 802.11n compatible. Use 2.4GHz for slower speed and longer range; Use 5Ghz for faster speed and shorter range.
Let me know if it works.