move the extender re580d closed to AP500. if no luck, contact TP-Link support.

i don't think that smart plug is smart to do that.

change the channel width of 5ghz to 40mhz for a try.

show us the screenshot? what is the current firmware? which one you have download? if you connect Xbox one to modem directly and do a speed test, what is the result?

try change the router lan subnet to 255.255.255.128. Then open a port 9 for ip 192.168.1.127.

do you have a Dell laptop with killer Wi-Fi? if yes, try this beta firmware. http://static.tp-link.com/Archer_D7v1_AU_1.6.0_0.9.1_up_boot(170407)_2017-04-10_09.31.50.zip If you still have troubles to...