Wi-Fi is usually served on two frequencies: 5GHz and 2.4GHz. Starry routers serve a combination of the two bands, and toggles back and forth between them.
The main differences between the two are that 5GHz is faster than 2.4GHz but can't travel as far. Likewise, 2.4GHz reaches further than 5GHz but is not as fast.
The dedicated 5GHz network is enabled by default. Just look in your device's Wi-Fi settings for your WiFi Name_5 and use the same password to connect.
Here's an example in which you might consider using a dedicated 5GHz network:
You have a gaming system in your living room. That system is in the same room as your Starry router, just a few feet away. Because a device like a gaming system uses a lot of data, and because it's in very close proximity to your Starry router, it would perform well on a 5GHz network.