Why would my APN change in my phone setting to MTS?

[AC Question]

I have a AT&T Samung Galaxy 4GS, that is now on StraightTalk. I rooted my phone and then set up a hotspot. Now in my More Network my APN name has changed to MTS and all the other setting has changed. Every time I change them back to the straightalk they go right back to the MTS. My WIFI does not wor but I do have data. What is MTS? How can I fix this?