A really odd issue that I (and a few others) can't work out...

[#1]

I'll keep this simple - for a few months I was hosting online game servers without any issues at all, then people online stopped being able to see the servers I'm hosting - this is despite my copies of those same games being able to see other online servers without any issues at all.
All the ports appear to be forwarded correctly - and anything connected to my router (whether by cable or Wifi) can see my servers as well as online ones - but nobody outside the routers range can see any servers I host at all - even with a static IP address set up. I've even tried hosting the servers on 3 different systems with no change.
As you can gather it's got myself - and several others - completely puzzled.  There seemed to be no reason for the issue to come up randomly without warning and I was wondering if anyone had any suggestions that could solve this issue.

[#2]

I take it nobody has a clue about any possible solutions then?

[#3]

Good morning Hugh,
A warm welcome to the forums.
I'd suggest you give the community a bit of a chance here.  You posted late Sunday afternoon and only 15 hours have passed since you posted your question.  It is not a generic issue and might be better placed in the games forum.  I've asked the mods to consider moving it.

[#4]

Quote from: hughcox77

I'll keep this simple - for a few months I was hosting online game servers without any issues at all, then people online stopped being able to see the servers I'm hosting - this is despite my copies of those same games being able to see other online servers without any issues at all. All the ports appear to be forwarded correctly - and anything connected to my router (whether by cable or Wifi) can see my servers as well as online ones - but nobody outside the routers range can see any servers I host at all - even with a static IP address set up. I've even tried hosting the servers on 3 different systems with no change. As you can gather it's got myself - and several others - completely puzzled.  There seemed to be no reason for the issue to come up randomly without warning and I was wondering if anyone had any suggestions that could solve this issue.

Nice to see that I'm not the only one!
I started having this issue about three months ago, possibly a month or two before; I'm not sure.
I was hosting a game server and all was working perfectly, then one day, it decided not to show up in the list to my friends (outside my connection). I hadn't changed anything so I couldn't figure out why. Restarting computers and routers had no effect so I was baffled.
I've just moved the servers to another address (both are on Plusnet Fibre) and all was working fine an hour ago. I tried port forwarding a new port but it wouldn't work - so I restarted the router and now no ports are forwarded correctly.
This is a major inconvenience as I use my internet connection to host a lot of services for myself, family and friends to use and it's a real nuisance when none of it works!!!
Note: I just phoned Plusnet about the issue as soon as I got it and was immediately advised that they cannot support me and that I should go to the Community Forum.

[#5]

So are you both using the plusnet supplied routers? Perhaps they are not the best (I gave up using the technicolor).
I assume these game servers are  are on  windows - maybe its firewall plus updates "changing things" could add to the confusion.

[#6]

wonder if it's related to udp blocking that seems to have arrived, though that could be me making stuff up - but might be worth looking at

[#7]

Has the Plusnet kiddies firewall some how been switched on? Worth looking in the Member Centre Portal just in case.

[#8]

What is this UDP blocking?

[#9]

Is there any possibility of this being related to one of the acquired IP address groups resulting in someone's out of date geo-location database believing that these hosts are not in the UK?
Such a thought could be 'marbles' - but potentially relevant.
@Hugh & Jencox
Can you look at your current IP addresses and post the first 3 octets here please?

[#10]

The external IP address is 212.159.116.XXX  (Naturally I've covered the last 3 digits)
The first time I contacted Plusnet about this they suggested that it was happening because I lacked a static IP (which may explain one of the server programs, but the other one auto-updates any changes on the global server list)
With what Jencox has said, I did notice a major downturn in people getting onto my servers around that same time - but I had put it down to people being on less often during the summer holiday period.  
The router I'm on is the standard Technicolor TG582n.  Sadly I can't adjust its firewall under the admin login (I get an "This level is protected and not editable" response) - but I have enabled "Game Mode" - and opened the ports relevant to the game servers - with no change on people being able to see them.  All 3 systems programs are indeed under windows - and have been tried on 2 different systems running XP SP3 and a third running Win7 (starter edition)
If it is indeed the router that's causing this, I would like some suggestions for relatively cheap ones that can handle the job.