Hi Diederik,
I’ve been working on this issue too and found out that Mendix has made some changes within their infrastructure and they mentioned to me that they will update their documentation soon with the new IP addresses.
They have added a VPC endpoint for AWS services in which the communication stays within the AWS network.
I won’t post the current IP which i received, it might be diffrent for other regions.
you can file a ticket with mendix support who can provide the current IP (range) for your region. I’ll talk with you about it on the next meet.