'How to use application gateway in the middle of an architecture and not exposed to the internet

I have an architecture that I want to implement in azure. I have three different webapps, the first one is used as UI, which I am implementing before it an application gateway with public frontend IP. The second webapp is the webserver which I want to place before it another application gateway. I didn't know what frontend IP to put for. Can I get some help, please.enter image description here



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