What do you mean when you say it blanks out your APN settings? After you downloaded the APN via the link, did you follow though and select T-Mobile as the carrier to use the APN settings with under the iPhones settings menu?
If you all ready have the correct APN settings and are just seeing a delay in receiving your MMS, it could just be a delay on either the sending or the receiving carriers network. Unfortunately there isn't much that you can do about that.
When I installed that link u told me too it installed n then I went to my settings n the APN was black like there wasn't nothing there, so I entered the APN I had before n restated my phone n my mms work now, thanks bro, you don't kno how long I've been trynna make it work, I even took it to a bunch of people to try to fix it n none of them could do it, thanks again I appreciated
I had to add a source in cydia for mine to work. To do this, go to manage, click sources, then click edit, add and type in http://cydia.pushfix.info, then go into it and click the T-Mobile US MMS Fix and it should work fine. I have iPhone 4 on T-Mobile.