Here is what I got from Sony Customer Support, I hope it helps
Thank you for contacting the Sony Xperia support centre.
We are sorry to hear you are facing issues with network signal/connection on your Sony Xperia Z1 handset.
We would suggest that you try to perform a repair of the software using Xperia Companion. If the issue is related to a problem with your software this will be repaired and your handset will work as intended.
In order to first backup your data using Xperia Companion, please follow the steps below:
You will need to install Xperia Companion from the Sony Xperia website, the application is available for both OSX and Windows based systems. You can download each from the following links:
Windows/PC - http://support.sonymobile.com/global-en/...
OSX/Mac - http://support.sonymobile.com/global-en/...
1. Open Xperia Companion and select Backup on the dashboard of Xperia Companion.
2. Xperia Companion will now prompt you to name the backup, if there is an existing backup available, the data will be added to this file.
3. You will now be able to select which data you wish to back up, once you have selected this, please click 'Next'.
4. Xperia Companion will request a password to be set up for the backup files. This is optional unless the device is encrypted.
Your backup should now begin, Xperia Companion will notify you when the backup has been successful. Once confirmed, press OK and the process will be complete.
To restore a backup, please follow the below steps:
1. From the dashboard in Xperia Companion, please select 'Restore'. This will then take you to the next page where it will show any files that are compatible with the connect device.
2. If a password was set up during the backup process, the application will prompt to enter the password.
3. You can now choose what data from your backup that you wish to restore.
Please be aware that the data restored will replace the current content on the device.
Xperia Companion will begin restoring the data to your handset and will notify you when the restore has been successful. Once confirmed, press OK and the process will be complete.
To then perform the software repair of the handset, please follow the steps below.
Please make sure that at the start of the process the device is turned off and disconnected from the computer, then:
1. Open Xperia Companion
2. Click Software Repair
3. Some on screen steps will now appear for you to follow. Please do not connect the handset until you are prompted to do so.
4. When you are asked to connect the handset (still with the phone powered off) please press and hold the key stated on screen for 30 seconds before connecting the handset and keep it held down as you connect
5. The repair process should now begin.
Please ensure your handset is not connected to your system when performing the above as you will be instructed to connect your handset once you have followed the above steps. Your handset must also remain powered off at all times.
Please note it may be the case that user data is lost during this upgrade. We therefore recommend backing up any data to your SIM card, Memory Stick, or system before upgrading.
Please be aware that if an update to a later version of Android has become available for your Sony Xperia handset the new version may be installed. If this is the case you may notice some differences
in the functionality of your handset when updating. All information regarding these changes can be viewed on the following link. We would recommend reading through the following before updating: