Can't connect to Xbox Live - Doing my head in!!!

[#1]

Despite reading the other posts on this issue i still can't connect to Xbox Live wirelessly, or otherwise (i haven't got long enough cable for a wired connection).  I should start by saying that i have no idea about networking or technical stuff, i just need to know where to go and what to click! 
I've had transient success after following Matt_2k34's advice on how to DMZ on a previous post, i could connect effortlessly for a couple of days and then back to square one.
When i test my connection, the Xbox either connects to my router but not to the internet.  Or, it will connect to the router and the internet but won't connect to Xbox Live.
This is where i'm at:
i've gone into my router settings and disabled the firewall.
I've gone into home network>devices and clicked on "assign a public IP address of a connection to a device" and assigned the unknown device 00:25:xx:e0:6e:xx (which i am assuming is my Xbox).  In the configuration settings for this device, under the 'Addressing' heading, it now says 'always use the same IP address: Yes'.  I assume this means its a static IP which i understand is needed.
Still the same problem.  Surely there must be a way around this?!  Is there anything else i can do?

[#2]

I'm in the same boat. Would love a guide that doesn't simply assume that the connection will work when going through the setup process (http://www.plus.net/support/broadband/broadband_gaming/xbox_360_connection_guide.shtml)

[#3]

have you opened your ports?
http://portforward.com/
selecter you router and follow instruction may help. hope it does

[#4]

Hi,
I'm a new Plusnet user and had exacly the same problem. I'm going to post my short but sweet answer to each of the 3 questions on this subject.
In short I joined Plusnet, had a problem with my X-Box, assumed it was Plusnet, phoned to cancel my contract to go to Sky, was offered a month free subscription but initially chose to cancel anyway.
I then had one last attempt and called my local computer repair company. He said it was not an unusual problem and spent 20 mins updating the settings on my Thompson router. It only cost £25 and now the X-box and 2 laptops (work and private) are working perfectly and have been for the last 2 months.
I called Plusnet and they honoured their 1 month free.
Hope this helps

[#5]

See reply 12 at
http://community.plus.net/forum/index.php/topic,91897.0.html