I had this exact issue about 2 weeks back here in Houston, it only happened in a certain area and turning on/off airplane mode fixed it over the short term.
I switched to the Tmobile BYOD sim since Im sure Houston will have PCS 3G here soon. I would recommend against any MVNO ATT sim, it seems that ATT wants people on their postpaid plans and if they aren't messing with the MVNO APN on purpose its extremely obvious that it's below a low priority to fix it when issues occur.
“The further a society drifts from truth the more it will hate those who speak it.”
― George Orwell