AltStore is very useful for IPA files install to your iOS Device.

How to install AltStore on MacOS
1 Download and install on MacOS.
2 Click AltStore, from Mac icon bar.
3 Select Install Mail Plugin.
4 Restart the Mail app after complete installing process, enable the Mail Plugin.
5 tap on mail from menu bar and select Preferences.
6 Tap as Manage Plugins.
7 Enable – AltPlugin.mailbundle – Apply and Restart Mail.
8 MacOS Catalina – Go to summary page in iTunes and enable Show this iPhone.
9 MacOS Mojave – go to iTunes and enable iTunes WIFI sync.
10 Tap app icon – Click install AltStore select your iOS device.
11 Enter Apple ID and Apple Password.
12 Wait few time to complete installing process and go to setting -> General – Profiles & Device Management – click trust button.
13 Click this app icon and use it install apps for your iPhone.
How to install AltStore on Windows
1 Download iTunes or iCloud for your Windows computer using Apple website.
2 Download AltStore for Windows.
3 Extract setup file and click file to start installing process.
4 connect your iPhone to windows computer trust with device.
5 Now open and enable iTunes.
6 Tap the app icon and click Install AltStore and choose your iOS device.
7 Enter Apple ID and Password in the aria.
8 Wait few time to complete installing process and goto setting -> General – Profiles & Device Management – click trust button.
9 Click this app and use it install apps for your iPhone.
How to Install Apps on AltStore
1 Select your apps and download IPA file.
2 Open AltStore app and go to the My app tab.
3 Click the plus icon (+) top left corner, now you can list of your downloaded IPA files, click now you needed IPA file.
4 You are first time with AltStore, when open you have to enter Apple ID and password.
5 it will start installing the app, it will take few time to complete installing process successfully.
6 Now it will seen in the My Apps, you can open from Home screen.