Your Time Capsule is not downgrading to 2.4 GHz at all, it is always producing both a 2.4 GHz and 5 GHz network at all times. That's why it is called a simultaneous dual band router. Wireless devices will automatically connect to either 5 GHz or 2.4 Gz based on their own capabilities and their distance in relation to the Time Capsule.
5 GHz signals are "faster" signals, but they are also "weak" signals, so you will only have much success with 5 GHz when wireless devices are in the same room or have line-of-sight to the Time Capsule. Once a device moves a room or more away from the Time Capsule, the device "sees" the stronger 2.4 GHz signal at that location and switches over to 2.4 Ghz for the stronger connection.
Unfortunately, once a device switched to 2.4 GHz, it will usually not switch back to 5 GHz unless you temporarily turn off the WiFi on the device and then turn it back on. If the device "sees" a strong enough 5 GHz signal, then it will connect to that network.
If you want devices to connect to 5 GHz, by using an option to assign a different name to the 5 GHz network, you can "point" or "force" devices to connect to that signal. The downside to doing this, is that you will also need to manually "point" devices at the 2.4 GHz if the devices are located a few rooms away from the Time Capsule.....because they may not be able to even pick up the 5 GHz signal at that location. Even if they can, the 2.4 GHz signal will much stronger and probably faster than the 5 GHz network.
Post back if you need a few tips on how to assign a different name to the 5 GHz network.