xt1528 enable LTE/GSM without computer

Switch Sims
switch to settings -> cellular networks
Pull down notifications-> Airplane mode ON ( trick is here switch to airplane mod from top notification bar not from setting menu )
Network App -> GSM / LTE (close app)
switch to settings
verify 4g (I always click on it, still shows CDMA)
NEtwrok app -> Turn on Radio
Settings -> click on CDMA -> Select 4G
Airplane Mode OFF


source XDA forum