RogueRoslyn
Got it.
Depending on the AT&T router, you may actually have two (2) 5ghz bands available. Need to determine if you have a dual-band router (2.4ghz and 5ghz) or a tri-band router (2.4ghz and usually two seperate 5ghz bands).
In either case, you need to verify that the 5ghz radio/band is actually turned off/disabled and not merely hidden/not broadcasting the SSID (network name). A setting to hide or not broadcast the radio simply hides the network name from discovery by devices, but still allows connection if the network name is known. (or if you use software tools/apps that can find hidden networks).
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Is your Roku device now working on your 2.4ghz band or are you still having issues with it? Some model numbers would really help with determining what your issue is or the contributing culprits.