Sending/Recieving MMS

Okay, so ive done some looking around on different websites and i have no clue whats wrong with my phone. I have recently bought the Samsung Galaxy s2 SGH-i727. I used to have a Samsung Galaxy s1. I have my 3G SIM in my Galaxy s2 now and its a 4G phone...will that affect it? Plus i have tried a million different APNs to see if that would work, and it hasnt. Im able to send/recieve text messages, but i cant get MMS. I also use AT&T GoPhone as my carrier, but i didnt have this problem with my S1...Please help!! It will be greatly appreciated!! Thanks!

What happens when you suppose to recive MMS? you got SMS informing you about that (usally happens phone is not configured right, network simply consider that your phone does not support it... at least this happens in my local Polish carries :p)? YOo recive it just can't open it? or you not getting any signs of getting it?

4G should not effect as your phone should also support 3G and 2G for fallback and roaming reasons, in general they operate like extensions of each other not to mention LTE does not support calls (yet?) so 3G handle it.