If you aren't familiar with the differences between the bands, just understand that 2.4 GHz is older and has some issues. 5 GHz is newer, and has its own issues, but is generally better performing overall for most people.
Smaller and older devices support 2.4 GHz only. Most devices these days are dual band, meaning they can connect to either. Most will use 5 GHz if it's available and a decent signal.
Around 2015, 5 GHz became much more widely used, and became more common on cheaper devices. Roku had included dual band on all devices except the entry level ones by late 2013.
For the past nine years, that has remained the case. The low tier, entry level Roku devices have been 2.4 GHz only. The Roku 1 and Roku LT were 2.4 GHz only. When the Roku Express and Express+ replaced them, they were 2.4 GHz only.
In one of Roku's weirdest naming decisions, the Roku Premiere and Premiere+ originally was dual band, and was essentially a Roku Ultra with a few bells and whistles missing. It was dual band. However, in 2018, they downgraded the Premiere and Premiere+ from a scaled down Ultra to a souped up Express. And it lost the 5 GHz connectivity, being 2.4 GHz only. Like I said, merely a souped up Express.
When the newer but primitive Premiere line was replaced with the Express 4K series, that had dual band, leaving only the basic Express as the last 2.4 GHz only device.
That finally changes on October 13, 2022. The Express will be upgraded and the new model 3960 will be dual band. For the first time, all Roku devices will be dual band.
Roku is late to the party on this. Other lineups by Apple, Google, and Amazon have been all dual band for a while. And with the recent behavior of ISPs that changed the 2.4 GHz settings on leased devices, many Roku Express users have had major issues with connectivity. The ISPs were wrong for just up and changing settings like that, but Roku was way behind on technology by not offering dual band on their top selling, cheapest device.
I'm glad to see it. Only I know people, and some people will say that Roku is simply trying to get folks to buy a new device by making the older one stop working, when it's actually the fault of the ISPs. Roku should have done this years ago. But, in a month, it will be done, and the issues will start to go away as fewer and fewer of the older 2.4 GHz only devices remain.
If you're thinking about getting into streaming, or simply looking to add an additional but cheaper device to a second (or third) TV, either do not get a Roku Express, or wait until October 13 to get one. Make sure it's dual band. That will make your Streaming Life so much easier.