Double NAT Problem?

[Light]

I just started setting up my new Netduma R3 and whatching a guide on Youtube on how to set up the netdumaOS by an italian guy. Under the WAN section in the settings he says that you need to put some things there if you use DHCP and to enable DMZ on your ISP router, all of this to prevent the double NAT problem. Honestly i tried to do that but i do not know exactly what to do, can somebady help? Ihave an FWA internet connection.

i would like to attach screenshots such as what the DMZ tab looks like in my ISP router settings but there are some ip adresses shown so i dont want top risk...

[Netduma Fraser]

Copy the WAN IP from the R3 WAN Settings page and enter that into the DMZ section on the ISP router

[Light]

i did so, and also on the DMZ tab i have this option called 

External Interface:
   - VLAN 100 - PPP - WAN Ethernet (100.108.77.19)  
   - VLAN 835 - PPP - WAN Ethernet (Static IPv4 Address not configured)  
   - PPP - WAN Ethernet (Static IPv4 Address not configured)  
   - PPP - PTM (Static IPv4 Address not configured)  
   - VLAN 835 - PPP - PTM (Static IPv4 Address not configured)  
   - VLAN 100 - PPP - PTM (Static IPv4 Address not configured)  
   - PPP - Ethernet over ATM, 8/35 (Static IPv4 Address not configured)  
   - PPP - Ethernet over ATM, 8/36 (Static IPv4 Address not configured)  

what should I select?
and also is there a site or way I can test if there is this double NAT problem?

Thank you so much for your time.

[Netduma Fraser]

You shouldn't need to select anything else. Not specifically that I am aware of but as long as you can get an Open NAT now through a console/on a game while connected to the R3 then you're good