The Time Capsule broadcasts both a 2.4 GHz and 5 GHz wireless signal bands simultaneously. By default, both signal bands use the same wireless network name, although it is possible to assign different network names to both bands.
Wireless devices will connect to either 5 GHz or 2.4 GHz automatically based on their capabilities and distance in relation to the Time Capsule. For example, if you have a newer Mac laptop and it is in close proximity to the Time Capsule, then it will connect at the faster 5 GHz speeds. But, 2.4 Ghz signals are stronger than 5 GHz signals, so if the same Mac moves a few rooms away, it will likely connect at 2.4 GHz, since that would be the stronger signal at that location.
A device that is only capable of connecting at 2.4 GHz will of course connect at 2.4 GHz. That would usually be the case with an HP printer.
Unless you have a brand new HP printer that is capable of working with both 2.4 GHz and 5 GHz networks, the HP can only connect to the 2.4 GHz band, so I'm not sure what you did when you say that you used a 2.4 GHz channel only, since that is all the HP could connect to anyway.....unless HP has just started to ship new printers that can use the 5 GHz signal.
