Can anyone advise on internet access for an old NSA310 NAS drive?

Hi guys.  Very technical user here in other areas of PC work but not network experienced in the slightest.  You network managers are Gods to me!  What is it they say?  "In the country of the blind the one eyed man is king."   

I'm trying to set up an older NSA310 so I can access my data on it when out and about from my home, I was thinking via FTP.  I do understand some of the principles involved but I'm having a heck of a job seeing where things are falling over.

I have a BT Smart Hub, (no, don't please ), the first generation I think, and that is anything but transparent to me to set up in this sort of area.
The NAS drive has a fixed IP address which works flawlessly inside the LAN and can be accessed as it should be from any device on the local network.
I have a dynamic DNS account with a hostname set up on No-IP and their client running to update.  That resolves to the correct public IP address when pinged from outside my LAN but the ping request just times out as there is still work to do with the setup.
UPnP settings on the NAS and the HH6 seem a mystery to me.  On or Off?  I have read conflicting info online about that.  I'm currently trying to switch it on in both the HH6 and NSA310 but I get refusals as 'no device supporting UPnP can be found on the network'.
I have tried to set up a rule on the HH6 to forward a particular port but I can't complete that as the other aspects need correcting.  I am assuming I should be using the standard FTP ports for this though I wondered if a custom port was a better option.

I can't help thinking this should be fairly simple but it just goes round in circles so I must be missing something pretty obvious.  Can anyone help me to sort out the blank spots so I can figure out why I can't complete this?

Thanks in advance for any help offered.  (Even mockery is a satisfying thing when you just need human contact!  )