RDS Gateway - how to allow a whole subnet or range of IPs for use?

I've setup an RDS Gateway to allow and control RDP access out of the company and it works. My issues is within the RAP Properties > Network Resources tab, where you add the allowed destination IP, I can only add IPs individually and not subnets.

How can I add a range or subnet e.g. x.x.x.10-x.x.x.x20 or x.x.x.x/24 to this allowed list in the RDS Gateway?

RAP Properties > Network Resources tab

The above formats aren't accepted. As a work around previously I've exported and modified the tsgateway.xml export, however I need to add several large subnets now and it's really a viable solution.

Any help would be appreciated.

Thanks