You can download Fortnite:
Epic Games Store on iOS/iPadOS
Once you’ve installed the Epic Games Store, follow the steps below to download Fortnite from the available list of games and start playing.
Note: If you haven’t already installed the Epic Games Store on iOS; follow the steps on How do I install the Epic Games store on iOS or iPadOS?
Tap on the Epic Games Store on your home screen.
Select Fortnite to install.
Tap “Get” to download and install it on your iPhone.
Altstore.io
Once you’ve installed the Altstore.io, follow these steps below to download Fortnite from the available list of games and start playing.
Note: If you haven’t already installed the Epic Games Store on iOS; follow the steps on https://altstore.io/#Downloads
Tap on the Epic Games Store on your home screen.
Select Fortnite to install.
Tap “Free” to download and install it on your iPhone.
Fortnite and Epic Games Store is only available in the EU region. For more information, visit this link: https://www.fortnite.com/news/fortnite-returns-to-ios-in-the-european-union