The reason for asking whether DNS-320L was using dhcp or static, was whether you may have incorrectly entered the wrong gateway IP address, but you said you can successfully access the dlink via eg. http://184.108.40.206:80, so that sort of rules out any incorrect settings on the Dlink NAS.
You could perhaps change it temporarily to DHCP to see if it makes any difference. I doubt it.
As you can access the Dlink NAS directly without using the Dlink Portal, then it does indeed look like a portal issue.
Your guess is as good as mine as to how the dlink portal work.
One would think there is an initial registration process which subsequently causes your DNS-320L to 'phone home' periodically, and notify the Dlink portal of its 'current' IP address when it changes. So when you use the portal on a web browser (I presume it is not an app), it therefore knows the IP address of your Dlink (Plusnet router) and permit your web browser to connect directly to it.
Have you tried re-registering your Dlink NAS?
Unfortunately, the Hub One doesn't offer DDNS. I don't suppose your dlink NAS supports any 'free' 3rd party DDNS providers if you cannot get the Dlink portal to work?
You can change the default IP address on the Hub One to 192.168.1.1 if you wish, but TBH, I'd be very surprised if it did fix the problem.
Which Talktalk router did you used to own?
I am probably wrong, but there a rumour that the current TT wireless Hub may be unlocked and permits use of PPPoE even though TT uses DHCP protocol.
ex-Plusnet (ADSL, FTTC) 2008-2023. now BT (FTTP) 2023-