Can't connect to Steam using Three Mobile Broadband

[JC]

I'm trying to help my Dad who has Three Mobile Broadband as his only method of connecting to the internet. Every now and then Steam is unable to connect using his internet connection. Whilst he was here before and had this issue, I connected his laptop to my wifi in my home and it connected straight away and then after everything worked OK until now.

I've read forums saying that some mobile broadband ISPs block ports required for Steam and I have been unable to find any workaround.

The laptop he is using is on Windows 8 64-Bit.

We have tried: -
Deleting the ClientRegistry.blob
Re-installing Steam
Allowing all Steam apps through firewall
Turing off firewall alltogether temporarily

He is not in a position to connect to someone else's internet at present.

Has anyone else had the same issue and know of some fix?

Thanks

[Satoru]

3 Mobile is well known to block Steam on their networks. The same with many other cellular providers. there's no real fix sicne the network itself is blocking the connection.