Could you please update your device to the latest firmware, which you can download here, and see, if the issue continues. When updating, please set Keep settings option to off. Be aware, that you will lose your current configuration.
If you have important configurations, and update does not solve anything, please make a backup before the update. That way you will be able to revert.
Also, if the issue continues to be present after device update, I would like you to attach a troubleshoot file to your question from the updated version device. Please, replicate the issue by sending a couple of SMS, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.