I am having a similar problem. I have managed to secure the unlock code for my "former" AT&T Samsung Solstice with Tmo SIM. I can make calls and send texts, but run into problems when I attempt to send a picture message, or access any web-based service.
I attempted to follow the information provided in the activation card regarding setting up a new APN, but for some reason my phone wants to default to the old ones (which I can not delete) even though the phone is unlocked.
Can you Advise if I am doing something incorrect that I can fix to make these services work.