WS(Vindows7) With GPRS modem---->INTERNET

In the WS i see that: IP the address GPRS of connection changes several times within a minute.
At this time I can open https://www.gmail.com and is normal work with mail.

At this time i can open and use https://sharepointMyFirma.ua . This is OK.

But i`m can not use: https://Gw2012WebaccessMyFirma.ua ( GWWebAccees2012 Didn't accept the password !!! ). This is a PROBLEM !!!

Only after changeover of GPRS of connection, by another (the IP address was set only in case of connection of the modem and was invariable connection all the time) - I have an opportunity to pass authentication in https://Gw2012WebAccessMyFirma.ua

Who can state the view of this sitatsiya ?

Serg