So for me, and maybe this is obvious to some, was the usage of the wrong sha key.
What you need to do is go to your https://play.google.com/apps/publish/ console
Go to Release Management then to the App Signing page. Copy the SHA256 key from there.
Go to https://developers.google.com/digital-asset-links/tools/generator and enter the sha key, url and packagename there.
Upload this file to https://example.com/.well-known/assetlinks.json
Make sure that the file is in the correct location: A folder named .well-known
.
Release your instant app