Model : Archer C9

Hardware Version :

Firmware Version : 4.1.0 Build 20160517 Rel. 35411

ISP :

I am trying to access my NAS locally using a DDNS service (xxx.synology.me). I've setup the correct port forwarding and everything seems to be working fine on external networks (e.g. 4G on my mobile device). I'm having problems using the ddns domain to access the NAS when on the same network (local).

I did some research and this seems to be because the router is not handling the nat loopback properly. From what I've read the latest firmware update was supposed to fix this issue.
I'm now running the latest firmware and I tried enabling/disabling NAT boost. Is there anything else that I'm missing?

Any help is greatly appreciated.

Thanks in advance.