Can't connect to mobile network on Sony Xperia Z Ultra
Try these
&lt;> there is a usually a text facility to send to your Mob. ISP and they respond with GPRS settings that changes mobile Internet settings to correct 
&lt;> Else call their CS. Go to network settings--> Mobile data --> Your relevant SIM Card and ask manually what each parameter is beginning from ARN name etc 

&lt;> Last one to try  - if sony warranty is no good , is to get to a full reset and 
 &lt;> V Last one  is a new ROM using flashtool and xperifirm .