Does your SIM card have a static IP address? When the router is rebooted, the cellular module has to initiate the data connection anew. In most cases this results in the mobile interface receiving a different IP address.
So my other question is, can you share what you specified in the static route? More specifically, did you specify a static gateway? Because if your SIM card gets a dynamically assigned IP on each reboot, the gateway field in the static route becomes irrelevant since the remote network cannot be reached via that old address. Therefore, the route does not appear in the routing table.
If my assumptions are incorrect, could you then perhaps share your router's Troubleshoot file so that I can analyze the configuration and check the logs? Be sure to download the Troubleshoot file after you reboot the router, i.e., after the route is supposed to have been loaded but wasn't - this way the reason why might be apparent in the logs. The file can be downloaded from the System → Administration → Troubleshoot page. Since it will contain your router's confiuguration files, you may want replace IP addresses and other things that you may not want to share in a public forum with dummy values before downloading the Troubleshoot.