URL Forwarding sounds like it’s an HTTP based redirect which I don’t think iOS would allow. I also noticed you said going directly to mydomain.com:1000 does not work. The only thing that’s possible is that you are using a self signed SSL certificate. If so, iOS application will not allow self signed certificates and will require you to get a legitimate one (eg. https://letsencrypt.org/).
Let us know if you are not using SSL and we can see what other diagnostics we can do to help understand this.