AT&T has gone through a confusing series of revamps on their streaming options, accompanied by an equally confusing bunch of name changes.
Their current offering seems to be simply "AT&T TV", which you can download to Roku from the Channel Store. I'd suggest subscribing directly with ATT on their website (I'm not sure it's available any other way anyhow) so that you will be able to access it via all platforms that ATT supports, Roku, mobile device, computer, etc.
Roku Community Streaming Expert
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