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To achieve optimal performance, make sure your iPhone is connected to the 5GHz network on your dual-band router. Dual-band routers offer two frequency bands: 2.4GHz and 5GHz. The 5GHz band provides faster speeds and less interference, making it ideal for data-intensive tasks like streaming videos or online gaming and updating iOS.
The SSID (Service Set Identifier) is the name that you've given to your router's network. Your iPhone will automatically connect to the network with the SSID you've selected on your router or modem. If you have both 2.4GHz and 5GHz networks set up with different SSIDs, you'll need to manually choose the 5GHz network on your iPhone's Wi-Fi settings.