Internet becomes troublesome after the changing username and password

Hi,I guess BSNL recently upgraded the speeds. I have had this bsnl connection for more than 2 years now and it had been working fine until when bsnl changed the username and password of their internet service customers here in Hyderabad. I guess this change has something to do with the recent upgradation of speeds and they've also probably made some changes to their settings, albeit on their side on not on customer side. After this even I started to have problems with my internet. It has started to work very erratically, it is absolutely unpredictable. Sometimes is may connect sometimes it may. I use a Belkin wireless router and my OS is ubuntu. Generally, the issues arise when I use wireless (in which case it now generally open websites although the net connection seems to be established) or when I connect my ubuntu desktop directly to the modem (in this case the service level varies from low speeds to connection time out, again the net connection is established). In windows directly connecting with the modem gives decent net speeds.Have any one else been facing such an issue. And does know why is happening and how to resolve it.My modem model is UT-300R2Uand router is Belking F5D7230-4 g wireless router.