i was not able to see the Airport Extreme (AE) data.
The AirPort Extreme was discontinued by Apple over 2 years ago. A few years ago, when it was still current, the WAN to LAN speed of the AirPort was about 350-375 Mbps in the Small Net Builder tests as I recall.
For the WAN to LAN reduction in AE setup as router, is the reduction in speed relative to the ISP speed or it is an absolute speed limit of AE ?
Top speed on the AirPort is usually about 350-400 Mbps when it is configured as a DHCP and NAT router. The AirPort simply does not have have the processing power to handle much more than that. Remember, this product was designed in the 2012 timeframe when ISP speeds topped out in the 200-300 Mbps range. Most were less than 100 Mbps.
As I mentioned before, WAN to LAN speeds are not displayed in the specification sheets for most products, so unless you can get a hold of that number, you won't know what kind of Internet connection speed that the router is capable of handling. If you can't find a given product in the router test chart, it is either out of date or it has not yet been tested.
can i see a list of connection (LAN and WiFi) on the network through AirPort Utility ?
If you open AirPort Utility and click on the picture of the AirPort, a small window will appear with some basic information about the AirPort. Wireless Clients displays a list of the WiFi devices that are connected to the AirPort Extreme at that time. The WiFi devices might appear with their normal name, as an IP address, or as a hardware or MAC address.
Do the same for the other AirPorts on the network to see the WiFi clients that are currently connected to that given AirPort at the time.
Ethernet connected clients are not displayed in AirPort Utility, a serious mistake that Apple never corrected. They must assume that the world connects using only WiFi.
On the Airport Utility, "Wireless Clients" > "connection" > "data rate" can be as high as 1300Mb/s for MacBook. What speed is this referring to pls ?
That is the theoretical maximum speed at which a WiFi device can connect to the AirPort Extreme. The actual throughput is usually about half of the number displayed. So, if you had your Mac connected to a WiFi hard drive like a Time Capsule, the Mac could talk to the Time Capsule at higher speeds over the "local" network because your Internet connection would not be involved when the Mac was simply talking to the Time Capsule to back up data.
But, this same 1300 Mbps impressive looking number is meaningless if you are asking about the Internet connection speed. That is determined by the speed that your ISP provides and the processing power of the network router, in addition to the quality of the connection between the Mac and the router.
The Ookla test displays the Internet connection speed that is available to your Mac at the time.