Hi.
It will make a difference if the router itself is not the DHCP server.
Now which mode is TD-W8961ND running on? In Bridge mode?
Except TD-W8961ND, do you have one more router?
Can you post the screenshot about the Device Info page(Status->Device info) here?

There is a server that handles the DHCP, when the server is off and just the router and PC on the network ports are still blocked. Thanks.

Hi,
What is the server handling the DHCP server?

When you turn off this server and connect the PC directly to the modem bypass the server, do you set a static IP on the PC, double check its port number and then do the port forwarding on the modem? And when doing port forwarding on the modem, do you select PVC0 for Virtual Circuit, go to Virtual Server, and then open a port?
And please make sure that port 515 is already open successfully on the PC.

Then to test if the port is opened successfully, you can check the WAN IP on the device info page, and then telnet WAN IP 515 on the remote PC from the external network.
This link is about how to use telnet command: https://support.terranetwork.net/web/knowledgebase/131/Using-telnet-to-check-port-access.html

There is a microsoft small business server handling DHCP. When turned off I had to assign manual ip addresses to access the internet.

I have selected pvc0. I can telnet the router externally but just pauses when I use the 515 on the end. As well as trying to use the port forwarding as I wish I have been using the website CanYouSeeMe.org to test to see if port is actually open.

I am assuming the way the router is setup is not allowing port forwarding but could the ISP be doing this ?

Have you changed the LAN IP of TD-W8961ND? If not, please try to change the LAN subnet mask to 255.255.0.0, then please check whether the port can be accessed.

Yes, LAN ip has been changed, subnet currently set as 255.255.255.255. Wll try this, thanks.