Hello,
Thank you for contacting us.
Did you uncheck Keep settings, when updating the firmware?
Could you try to reset the device to factory defaults?
Could you also check if reverting to the previous firmware makes the modem visible again?
I would also like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.
Best regards,
Žygimantas