Why isee NO SERVICE on my recently unlocked CRICKET OVATION 3 / U668AC / ATT MOTIVATE MAX / U668AA

First go Settings -> Network & Internet -> Mobile Network -> Advanced -> Allow 2G service and enable it ! Also do note that  this phone support a limited set of bands so be sure that your operator supports it. Supported bands are the ones bellow:

GSM MODE (EGSM900 / DCS1800 / PCS1900 / GSM850)
UMTS MODE (WCDMA-PCS-1900 / WCDMA-AWS-1700 / WCDMA-CLR-850)