Firebase dynamic links are not working on iOS when the dynamic link has the custom subdomain

If custom domain not working and Google domain working then you need to add FirebaseDynamicLinksCustomDomains key to the info.plist for iOS

<key>FirebaseDynamicLinksCustomDomains</key>
<array>
  <string>https://custom-domain.com</string>
</array>

Ref: https://firebase.google.com/docs/dynamic-links/custom-domains

It's because I was using FirebaseDynamicLinks 1.4.0 which is old. When I update to FDL library 3.0.1, dynamic links with custom subdomain are working fine in the app.

The reason why I believed that I was using the latest FDL library and couldn't identify that I was using old FDL library is, CocoaPods(a dependency manager for iOS projects, read more) couldn't get me the latest version of FirebaseDynamicLinks for some reason, no matter what I do like remove and reinstall FirebaseDynamicLinks from Pod file or run pod update command. So, I have removed FirebaseDynamicLinks from pod file and ran the command pod install, it removed that library from my project and now I have downloaded FirebaseDynamicLinks framework from firebase console and integrated it manually on my project. Now with the new FirebaseDynamicLinks SDK, dynamic links with custom subdomain are working fine in the app.