Assign Groups From Trusted Domain to IAM Role Not Working

Hi

I have setup the AWS Directory Service and have a successful outgoing trust relationship to my on premise AD domain. I can assign permissions within my RDS instances, for example, and logon to them using my local, on premise, AD credentials
I'm now trying to get AWS Management Console access using our on premise AD credentials working
I've enabled Management Console access, created an IAM role with a trust relationship to AWS Directory - it shows up in the Delegate Console Access box within DS config
Problem - when I click on the IAM role and within Manage users and groups for this role I choose Add - all I see in the drop down is my AWS Directory Service AD domain, I can't see my on premise AD domain in order to select Groups from there
What am I doing wrong please ?

Thanks
STEVE