I believe there's an app that lets you install unofficial apps without being in the app store, sadly its a chinease app and you need to install it in the computer, connect the iphone with the computer and install using that app.

talking about connecting i believe you can install it in your computer and connect your computer to your phone?

or maybe you can load the app in xcode in computer and load it into your phone using xcode in yoyur computer, i did this once. sadly xcode is 9gb and takes like 3 hours to install. probably an overkill