Nano-Sim Card Transfer

I am trying to take the activated nano-sim card from the cheap Android flip phone AT&T provided me when they stopped supporting 3g and use it in a iPhone 7 Plus. A little research claims that if the SIM from the Android is the same size as the one the iPhone uses, it’s simply a matter of removing the activated SIM from the Android, putting it in the iPhone and voila! the iPhone works. NOT!! I cannot download and use the Android app “Move to iOS” from “Google Play” because it is not compatible with the POS Android (which utilizes the KaiOS platform, which I’ve never heard of). The app from the App Store does not have it. The iPhone 7 Plus is ready to set up as a new device and from what I’ve read, all I need to do is put the SIM from the Android into the iPhone and I’m in business. It doesn’t appear to be that simple. The flip phone does not pick up a cell signal at all, while the iPhone gives me at least one bar, sometimes two.

I hope someone can offer me the benefit of the Community’s expertise. I’d genuinely appreciate the help!