Hello,
I did try SMS sending from RUTX14 with RUTX_R_00.07.04.2, but did not encounter any issues.
Do you have a SIM from another operator to test, whether the issue also replicates?
What happens after the freeze, is it temporary, or do you have to reboot the device?
Would it be possible to generate a troubleshoot file from the device, while the issue is present or at least before rebooting the router? To generate the file, access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
The file can also be attempted to get via SSH, if the WebUI is not responsive, as instructed here in the answer.
Attach it by editing your question.
Best regards,