Netgear Nighthawk X10 / R9000 won't recognise admin pass & pass reset doesn't work

Update:

I believe I've found an odd solution to the password thing. So, I factory reset the router again via the pin-hole on the back. Then set the router back up via routerlogin.net via the on screen instructions. Now, this is where I'd eventually get logged out without any success of being able to log back in. What I did this time, however, I went over to admin settings and did another change of the router login password there. Then at one point I believe the "log back in" screen popped up and with the new password I was now still able to log back in! I hope this stays that way and I no longer have to reset the router again to gain access to admin page lol.

About the Vodafone situation - I rang them just a few moments ago and by explaining pretty clearly what I was looking for (guidance within Static NAT/DMZ settings) I was told that he'd be unable to assisst as he doesn't have the knowledge there nor would anyone else if I was to get connected to someone else supposedly. Well, fair enough, let's believe him for now. The truth is -  don't think there is too much knowledge around it anyway as far as I can tell - there is a flip switch (on/off) and an IP address to be put in. If I put my few brain cells to work they tell me that the IP address it is asking is that of the R9000 router - so, I entered it there. Would you say there is possibly anything else that would need to be done to eliminate double NAT here or this would be pretty much it?

EDIT: About the password thing - coincidentally I might have done the double NAT thing and password reset inside the admin page at around the same time or in between factory resets. Doing the NAT settings inside the Vodafone router I think you are right - sounds more likely to have fixed the password issue. 

Thank you greatly for your help!