You can always revert your tcp/ip config back to your ISP default, if Google DNS also doesn't work.
Sometime back I was not able to connect to my companies corporate VPN server when using Vodafone 3G, the problem got resolved using Google DNS, hence recommended this.
I don't see why an ISP should have a problem with allowing a different DNS, I am not sure if the ISP will even know this (not 100% sure, not an networking expert :ashamed: ).