The gateway cannot connect to the specified domain

Hi, I've got a Storage Gateway configured in AWS, and a local software appliance installed to a VM in Hyper-V on Windows Server 2012 R2. I'm trying to join the gateway to the domain, but not having much luck. I am seeing " The gateway cannot connect to the specified domain".

I'm trying to work out how the port requirements affect this. HTTPS is handled by the firewall inbound and redirected to a website hosted on site. Is this likely to be stopping the gateway registering? I've read the port requirements document (https://docs.aws.amazon.com/storagegateway/latest/userguide/Resource_Ports.html) and it does say HTTPS inbound is required, but am wondering why AWS would open an inbound connection? Doesn't the VM initiate the connection?

Any other things that I need to consider?

Thanks