Really quickly: The NodeBB folks favor Ubuntu. The CentOS guide you linked is pretty dated, as evidenced by some now defunct links. I did not use that one so cannot speak to it. It may perhaps be more expedient to move what you have to some backup dir and do over following Scott's guide. I know that works. Moreover, Centos 7 default repos bundle really dated packages. Recommend you set up nodejs and nginx repos if not already.
If you have gui access on that box, I would be curious as to what happens if you bypass the proxy and jack straight into NodeBB on port 4567? If not and you want to try this from an external network, I recommend configuring firewall such that only YOUR ip is allowed to hit that port.
If it then works, then something is amiss with nginx config (wh/I do not have time to review just now.). If not, then more likely NodeBB conf issue.
Its your boi sharlet here again with another problem lol. So I have setup an forum recently but I cannot register there the only way to acess the forum as an user for me is to login as the admin rather than that no way of registering!!
I have tried to ask @PitaJ ( Global Moderator ) as he helped me alot with my forums but sadly hadnt recived an reply!
It would be great to know why am I experiencing this issue!