Alarm Pro and Eero Pro

Okay I am really confused. I bought the alarm pro and the instructions in the booklet say to make the alarm pro the gateway but within the setup app instructions I'm not being offered this option at least clearly enough for me to understand. Please help me setup all this. Alice PS--okay I think I'm seeing now that I don't need my old eero modem. But I still don't understand the in app instruction choices. I don't understand which one to pick to make the alarm pro the gateway