Connectivity Issues via DDNS/VPN with RT-AC87U

Hi,

I am having an issue where I cannot connect to my network using a different ISP.
I have an RT-AC87U and have setup DDNS with access through WAN enabled as well as running a PPTP server on it.

I had an RT-N66U and only last week upgraded to the RT-AC87U.
This is strange to me as I am able to only connect to my DDNS and VPN when I am at another location but using the same ISP.

I have tried 2 other ISP's and have been unable to get access.

The RT-N66U is now connected in my second...

Connectivity Issues via DDNS/VPN with RT-AC87U