I think only ones on 2.4ghz only are the express and premiere models.
Yes. Look on the Wiki site listed above. The network capabilities are listed towards the right of the chart. If it's green, it's dual band so support both 2.4 and 5 GHz. If it's yellow, it's 2.4 GHz only.
Please don't say 5G when referring to WiFi. 5G is a cellular network protocol, not WiFi. For home networks, it's 2.4 GHz and 5 GHz. 5G is not the same thing. (lesson over )
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.