Upgraded my plan from Performance Starter (25mbps) to Performance (100bmps) - but speed is same.

But here is the most interesting thing:

if I do a speed test on speedtest.xfinity.com - I get the full 100bmps

If I do the speed test on speedtest.net - I get the old speed. 

If I do the speed test on any other 3rd party speed test website, I still get the old speed.

I've performed the speed test on various devices - Laptop/Iphone/Samsung TV

I get same results - on speedtest.xfinity.com - close to 100mbps, on any other website - not more than 20-28mbps. Samsung TV via Netflix app doesn't go more than 16mbps.

I've been trying to speak and explain to Xfinity tech support, but they all tell me that My Wi-Fi is bad. If my wi-fi is bad, how come I get the 100mbps on speedtest.xfinity.com then, through the same wi-fi connection?

I factory reset the modem, not helped.

I'm not sure what might be the problem, but I get more frustrated each time when I call tech support...and as of right now, I already called over 4 times during the past 2 days...