Can't access network from outside

I just moved from Ziggo (where everything worked).
My KPN router is in “modem mode” and just passes through to my Router (Asus RT-AC66R).
Port forwarding is enabled and set up for the services I want to access.

I use a DDNS service (no-ip.com). When I try and register this with my router I get the following errors:
> The wireless router currently uses a private WAN IP address.
> This router may be in the multiple-NAT environment and DDNS service cannot work in this environment.

If I Google my external IP and try to use the services outside of my network (http://external-ip:port-number) it just times out.

Can anyone help?