Hello,
Did you solve the issue? If yes, please share with us what did you do to solve issue. (Maybe it'll help others)
If don't, please try with these steps. (once at time)
1) Restoring the device
2) Upgrading the device firmware
3) Test with other SIM card.
4) Disable Wan failover for testing support
4) Edit the file /etc/resolf.conf and add the line nameserver 8.8.8.8
If none of these steps solve your issue, please go to System -> Administration and download the troubleshoot file.
You can send a private message and we can take a look to this.
Please let us know how was going with it, regards.