On running Network Diagnostics : [You must login to view link]
Router Settings : [You must login to view link]
yet I am still usually FP : [You must login to view link]
Tried with both uPnP on and off.
Help would be very much appreciated!

hi
use this free app and see if 16000udp is indeed open: [You must login to view link]
the dg-hr3400 is not a modem/router. what is your modem make/model number?

You are right, the 16000 UDP is not open. [You must login to view link]
DG-HR3400 is my router
Do you mean [You must login to view link] (Settings for my ONT(Optical Network Terminal))

hi
ok
it looks to me as if your set up is wrong. confirm your cable in to the house goes 1st to the ONT, then a hard wire to the hr300.
every connection to the net via a isp requires a separate modem or modem /router combined. the modem part is what logs you in to the isp with name/password. most cable modems are just that, no advanced features, and the hr3400 router would have ppoe as the way it connects.
from the pic you show, your ONT has all the features of a router as well as being a modem. p/forwarding etc. so your p/forwarding is in effect going thro' two routers.
the ONT needs to be bridged or the hr3400 turned in to a switch [putting it in to the same ip subnet as the modem etc. easy to do]
to test if im correct, disconnect the hr3400 from the pc and from the ONT. now directly connect, hard wire, the pc to the ONT and try.

if you look at the p/forwarding you have done you will see the ONT is 192.168.1.?. the hr3400 is 192.168.2.?