You could diable NAT and assign your public address as your router's address. For instance, if your WAN Ip range was 1.1.1.1/29 with 5 useable (excluding your WAN IP), in the basic setup under net work setup you would enter the following.
Local IP address: for example 1.1.1.7 which would be the same as your router's public IP.
Subnet Mask: 255.255.255.248
Gateway:1.1.1.7
DNS: you may need to enter your ISP's DNS server/s
Once you have saved this you then need to disable nat by going to Advanced routing and selecting "Router" under operating mode. Save and apply
If you've left DHC enabled all the network devices connected to the router should be assigned one of the 5 available public addresses (1.1.1.2 - 1.1.1.6), otherwise you can manually assign these on each device.
Ofcourse another option would be to use PAT and translate alternative ports such as 3390,3391,3392, etc to 3389 and send these to the relevant private IP's.
Now if someone could help me set up routed with NAT on the same firmware I'd be a very happy man :D