Can't connect to mobile network on Sony Xperia Z Ultra Device: Sony Xperia Z Ultra 
Carrier: The Peoples Operator (Same issue when tested with EE as well)
Android Version: 5.0 Lolipop

I recently lost the SIM card tray for my device, and Sony sent me out a new one. I inserted the new tray with the SIM, however now I get no signal. I occassionally pick up signal in a few certain places in the center of town, but that's it. 99% of the time it just shows an empty signal bar. The thing is, if I go to Settings>Mobile Networks>Network Operators and search for networks, it will find multiple networks most of the time, including my network (TPO) however, when I click to connect, it just says 'Registering on TPO' and just stays on this screen with the icon spinning.

I had this issue with an EE SIM, now I'm having the same issue with a SIM from The People's Operator, so I don't think it is the SIM. I've also tried two SIM card trays, one from Sony and one from a third party seller.
Any ideas why I can't get any signal? Getting Desperate now.

Don't think I can send it back to Sony even though I've only had it a couple of months as I had the screen repaired, which I'm guessing voids the warranty.
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 .