My nephew dropped my Xperia XA1 back in July and sent it to Sony for a repair. I had to place the sim card back in along with the sd card and had to do a fresh install. Try backing up using either the backup and restore in the settings menu or use Xperia Companion on the PC (find this better as it backups app / game data in full) then when done go to settings and system then tap reset phone and yes to deleting all from SD card and you should be good to go.
**side note have you ran the phone in safe mode to see if the network issue persists. **
