there is not a way to change mms settings in lg c900. Usually the phone only works mms or internet with the carrier it was locked too. You may use a slower internet connection, texting, and calling.
I had a locked At&T phone, unlocked it to use with t-mobile. I can call, text, but no fast internet connection or mms b/c they do not support the particular phone and this phone does not have settings for mms