They're wrong. The current Express and Premiere are 39xx and don't support 5GHz. Of the current models, the Streaming Stick+, Ultra, Streambar, and Soundbar are dual-band. Assuming you're in the U.S., you can see the current players here: https://www.roku.com/products/players
When you look at a product's details go down to "Networking" and look for "dual-band wireless".
There may be older models out there on a retailer's shelves that also support 5GHz. I don't know about the TVs.
As a final note, you don't need your Roku to support 100Mbps. None of them will use that much speed through the Internet. Locally maybe, but not via any of the common services. You might just want to stick with what you have and only get new Roku if necessary. That 100Mbps is more of a gimmick than anything. I doubt you'll notice any difference in your Internet speed.
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