Router -->Router-->NAS - how to allow access from WAN?

I am using a complicated setup with a 4G sim in a Huawei B593 router.

Connected this to an Asus RT-N66U and using that for home LAN access.

I am using a VPN service via the Asus device as it avoids a lot of EE congestion issues and it works just great.

My home LAN is not accessible through the internet however although port 80 is open.
I want to access my NAS which is possible via port 8080 but this cannot be done even if I put the NAS device in the DMZ of the Asus router VPN. I tried port forwarding 8080 also but still no joy. Any clues as to where I am going wrong?

Most likely EE are using carrier NAT and therefore inbound connections are not going to be possible.

What address is your Huawei router picking up from the WAN?

The VPN tunnel has a fixed IP address so I thought that would do away with that problem?
Inbound connections on port 80 are possible so I don't get why port 8080 is not.