unlocked AT&T phone will not allow T-Mobile MMS

I recently purchased an unlocked AT&T LG Neon. My service provider is T-Mobile, and after spending about 30 mins on the phone with tech support, they finally decided to tell me that because the Neon is "technically" a smartphone (I'm not sure I see how, but whatever), it won't send/receive MMS without an internet data plan, which I do not have. Is there a way around this? I personally find it ridiculous that because it won't let me change the connection setting from AT&T to T-Mobile, while EVERYTHING ELSE is correct, I can't use my phone for MMS.

Help? Pleaaaase?

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Is there any way you can get the T-Mobile info. settings and input the information manually?
You have to set the wap settings, the GPRS settings and last the MMS settings, if you can manage to do this manually, you could be able to send and receive MMS.
This same procedure is the one you use to set the wap settings in a clone phone.