Setting up a 2.4g and 5G network for devices.

I am trying to use my temperature monitor and home cameras and just recently I have upgraded my router and modem. Subsequently, neither work because they use 2.4 GHz and I the Wi-Fi signal is being sent out at 5.0GHz. I tried to use the app to change my phone to 2.4GHz, but then the network locked me out and would not allow me to use the 2.4. How do I set up my devices if the router won't allow me on the network over 2.4G?