When I set up my plug, I had to disable my MAC blocking while I was going through the install. As soon as I got the plug online, I was able to read the MAC address on the device table for my modem as well as my alexa app on the fire tablet. I added the MAC to my list of approved devices and then enabled the MAC blocking on the router.
I find it helps to have multiple screens available so that I can keep the modem controls open on a PC and then do the installations from either a phone or tablet. My PC and modem are in the same room, so when I'm setting up new alexa devices, it helps to do them all in the office and then move the device to its final location after it's all set up. Once you know the MAC for your plug, write it on a strip of paper and tape it to the plug for future use. ;-)
Android phones can download any of the wi-fi analyzer apps on the Google Play store. These free apps will read the MAC address of a smart plug if it's in setup mode. Not sure iPhones have similar apps.
NOTE: I do NOT speak for or represent Amazon in any fashion. This is a customer to customer forum.
Disclaimer: I have examined any sites I provide links for and have determined them to be safe but I make no promises about safety, content, ads served or anything that might make you clutch your pearls in horror. Use discretion clicking on links provided by total strangers such as myself.