The issue seems to be related to mobile connectivity than the VPN itself. Initialization sequence successfully completes, but then mobile disconnects.
Do you have Failover enabled, with mobile connectivity having higher priority than wireless?
Would it be possible to get a complete troubleshoot file?
What firmware was installed before update and which firmware have updated to?
Was the connection stable before?
Did you update with Keep settings enabled? Have you tried device reset to factory defaults, reconfigure and test it afterwards?
Are you using Auto APN? Could you try custom option?