It sounds like you're following this page: BYOD T-Mobile data & APN settings which is fantastic! If you have input the settings as mentioned in the link, we don't have any additional recommendations as MMS usually isn't supported on internet only devices . However, to answer your question, generally we would leave the username and password blank.
I have the same problem with a GSM / Gprs modem. The settings for MMS under BYOD do not work! BTW, SMS works fine.
APN: epc.mobile.com, MMSC:http://mms.msg.eng …wapenc, everything else is left blank. Can not set MCC or MNC, but could set a MMS URL if required.
This is a basic (dump) GSM device. Need correct setting.
Please take a look at the post above and let us know if those settings help. Thanks!