So I recently moved into a place where they have Spark Fibre with Huawei HG659B for internet.

I have a Ubiquity ER-X as a router/switch that I had for the main router device at my last home but I can't unfortunately swap this one with the family I am currently living with. Also, a Unifi AP AC Pro as my AP where I used to run a few Wifi networks for IOT and my main network also a guest network with Hotspot service. In addition, I have a raspberry pi which I use to run Unifi Controller and pihole.

I am wanting to access the Unifi Controller and pihole interface from the internet as a basic setup to begin with but I can't get pas the HG659 as it is not letting forward any port nor it is responding to any of the dmz config.

The ER-X is currently running on a LAN port of the modem, with it's own internal network - using HG659 192.168.1.0/24 network as it's WAN connection. 

So, now I have tried all that I could - to put the ER-X in the DMZ, port forward ports to the ER-X and then from ER-X to the Raspberry pi, creating Port Trigger apps but none of them is working. I have tried to check open ports on yougetsignal but they all come as closed.

I know that ER-X is responding to the port forwarding requests as I can access the internal services behind it, using it's WAN IP - (the one in 192.168.1.0 range - using HG659's wifi). 

Can anyone suggest me anything I can do to get this working because I can't think of anything that can make this work inside the HG659. I have tried to run HG659 in bridged mode and configuring ER-X as the PPPoE session but that doesn't get authenticated from Spark as I am not too sure of how that setup might work too.