MS RDP to internal workstation not working

Hi guys first post here, and I've been racking my brain on how to resolve this but am hitting a wall.

I'm basically having trouble using RDP from my study machine accessing the server downstairs. It just doesn't seem to want to accept the user credentials, however these same credentials work fine on other local machines to access the server.

I've attached an image of my network rather than trying to write it to explain.


So basically my Server is running Windows 8.1 Pro with remote services turned on and firewall settings configured

Study PC is also running Win 8.1 pro as above

Study is setup to to accept RDP sessions on port xxxx

Server has had it's registry updated to allow RDP sessions over a different port

All configured in the router on the port forwarding rules

The Study PC can be accessed via internal network remote sessions on Mac and iOS devices. It can also be accessed from the external network from my workstation at work

The Server PC can also be accessed via internal network remote sessions on Mac and iOS devices with no issues, and also from my workstation at work.

However, from my Study PC I simply am unable to access the Server PC via RDP at all. It just won't accept any user account credentials I enter into the log on info.

I've checked port settings. Windows Firewall settings for incoming and outgoing traffic rules, on both machines.

I have tried the following in the computer box on the RDP windows log on client

servers local IP in format [ 10.0.1.1port)]
WAN IPort

I've also setup DNS records for my domain name for RDP access so as not to have to type in an IP

eg hostname.mydomain name points to -> WAN IP

This method works fine from external network as well by the way.

It's got me completely baffled. Hoping a fresh pair of eyes can offer something, and hopefully all the above made sense.

Firewall rules would be why. Easier to just use Teamviewer, since that is what everyone is going to, instead of RDP, which to this day has a lot of issues, that Microsoft will not fix.