Was the dongle a completely different provider of internet services?
If so I think we can 99,9% exclude issues related to ISPs, and it's gotta be more something about your PC.
Still, being a specific issue (i.e. that affects only a very specific part of the internet, not all network communications) I doubt it can be something general related to your PC settings.
The closest thing I can think of that could produce selective results like the ones you're experiencing is a strange settings in Firewalling software.
Do you have a firewall installed? (Windows Firewall? Others?)
Do you have other similar software that control or simply monitor network data?
Do you have some Internet Security suite/software? Some antiviruses have some similar built-in functions.
Can you think of anything fitting the description?
Because other than that I'm clueless as to how helping you any further...