As far as I know there haven't been any DIY mixing apps on Apple app store for quite a few years and there never will be. So you have 2 options...
1. Use an online calc like ELR or Steam Engine in a browser on your Iphone.
2. Buy a cheap, used Android smartphone or tablet with Android version 9 or higher to use as your portable mixing calc. Connect it to wifi, go to Google Play store app and install the app called LiqCalc. There might be others but that's what I and many Android users use. I'm surprised Google/Android hasn't put an axe to DIY apps yet, cuz y'know "vaping is bad, m'kay".