Trying to connect to my work VPN, the VPN works fine on my home WIFI but trying to connect using my 4g dongle it fails.

In the TP-Link router settings I changed:

NAT Forwarding > Virtual Servers and added the port forwarding details (1723 and IP: 192.168.0.21 with TCP)

-This should permit TCP network traffic on port 1723, right?

And then I turned off UPnP

But when I try to connect my windows laptop it says "Unable to connect - The VPN connection between the computer and the VPN could not be established. The most common cause for this failure is that at least internet device (for example, a firewall or a router) between your computer and the VPN server is not configured to allow Generic Routing Encapsulation (GRE) protocol packets."

Reading online it seems I may need to configure the network firewall to permit GRE protocol 47? How can I do this? Any advice please?