I'm sorry to inform you that if your phone's WCDMA is 850/2100MHz it will only get 2G service.
I should know as I have a phone with 2G: GSM 850/900/1800/1900 MHz, and 3G: WCDMA 850/2100 MHz. I live in the Tampa area that T-Mobile has already added 1900MHz to MCDMA's HSPDA band. I too had thought I'd get 3G and even maybe 4G, but it doesn't.
Understand you stated your 3G band frequencies on your phone are 850 and 2100MHz and unless a service provider adds 850 or 2100MHz to THEIR 3G system your phone is not capable of getting 3G because it's 1900MHz is in the 2G GSM band.
Sorry to have to tell you this but it's a fact, I'm not happy about it, but I sell cell phones and it's not easy to find 1900MHz in the MCDMA group.
Hope it helps the confusion,
Rick