How do you setup your smart home Wi-Fi device if it only supports 2.4 GHz networks and your Wi-Fi router uses the same SSID for both the 2.4 GHz and the 5 GHz Wi-Fi networks?

Your phone with the app for setting up the smart home device will not be able to choose between the 2.4 GHz and the 5 GHz Wi-Fi networks since they both share the same SSID.
Your phone will most likely prefer the 5 GHz network, and because of this the app on your phone will not be able to setup the smart home device.
I had this problem when I wanted to add a Sonoff ZigBee Bridge to the eWeLink app.

One solution to this problem is to walk around your home and try to find a spot where the Wi-Fi signal strength is low and see if your phone has switched to using the 2.4 GHz network since this has a longer range than the 5 GHz network. If your phone switch to the 2.4 GHZ network you can do the device setup from that location.
If you can't find such a spot where your phone switches to the 2.4 GHz network you can use the method described below to setup your 2.4 GHz smart home device.

You will need a laptop or another phone that you can use to create a mobile hotspot using 2.4 GHz.

Use the Wi-Fi network administration website or app (e.g. Google Home) to change your Wi-Fi network SSID name by adding a number to the existing SSID name (e.g. from MyNetworkSSID to MyNetworkSSID2).
Connect your second device (laptop or phone) to the renamed Wi-Fi network MyNetworkSSID2.

Open the settings application on your second device (e.g. a Windows laptop) and create a mobile hotspot (the process is similar on your phone):
Open your Settings application (WinKey+i), click on Network & Internet, choose Mobile hotspot.
Turn Mobile hotspot on and share your Wi-Fi connection:

• Set the mobile hotspot name to MyNetworkSSID (your original network SSID).
• Set the password to the same password used by the MyNetworkSSID network.
• Set the network band to 2.4 GHz.

After a while you might see that a list of connected devices is being populated.
Windows has a limitation of max 8 connected devices on a shared mobile hotspot.
If necessary, you can temporarily turn off any connected devices so that it will be possible to connect the phone you are going to use for the smart home device setup and the smart home Wi-Fi device to the mobile hotspot.

When you have connected the phone you are going to use for the smart home device setup to the mobile hotspot and has at least room for 1 more device to connect:

• Open the eWeLink app.
• Click on the Plus icon to add a new device.
• Click on the Quick Pairing option to add one device.
• Select the mobile hotspot network (MyNetworkSSID) and enter the password for connection to the network.
• Click on the Next button.
• Press and hold a pin in the reset or pairing button hole in your Sonoff device for 5 seconds until it starts blinking rapidly to indicate that it is in pairing mode.
• It should now be discovered by the eWeLink app and you can use the app to administer the new device.

Turn off the shared mobile hotspot on your laptop or second phone.
Use the Wi-Fi network administration website or app (e.g. Google Home) to change the temporary Wi-Fi network name back to your original Wi-Fi network name (e.g. from MyNetworkSSID2 to MyNetworkSSID).
You can now turn on any devices that you shut off earlier.

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