There you go, Chums....Sorted!... :)...!
After 2 whole days of fartin' about with my new MT 882 Modem/Router, (from my new Internet Provider), trying to open all those hidden Ports.....with no success... >:(...!
I finally abandoned the bleedin' thing, and re-installed my old, trusty Thomson ST330 Modem
(NO ROUTER!), (using the New Service Providers Installation Disk), and lo and behold no more Port Problems.
Back on-line again, and a quick check showed that all the Ports were open,
every single one!, allowing me to join any Multiplayer Game with out the dreaded Port Problems again....

...!
After wasting 2 days of all my aircraft being grounded, my FS 2004 and FS Navigator are up and running smoothly again.... [smiley=thumbsup.gif]...!
So there you go.....if. like me, you are a single Internet user, just connected directly to the Phone line via a simple Wired Modem, there is no point in having a complicated Router, with all its attendant problems!
The only problem is, simple Wired Modems are difficult to find, the market being swamped with fancy Wireless Modem-cum-Routers...

...!
F.......Back on Mulitiplayer Line again!....Happy Bunny!... [smiley=2vrolijk_08.gif]...!
Note: I still get the same Download/Upload speed* in the old Thomson ST 330 USB Modem as I did on the Fancy Ethernet MT882...

...!
* < 6 Mbs....

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.