This was a missing piece for me to make the serving endpoint work with the instance profile. The JSON below needs to be added to the IAM role trust which is used for the instance profile:

{
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "arn:aws:iam::790110701330:role/serverless-customer-resource-role"
                ]
            },
            "Action": "sts:AssumeRole"
}