You could enable logging to Flash. This will preserve the logs even after a reboot. This can be done by navigating to System → Administration → Troubleshoot and changing the "Save logs in" option to Flash memory.
However, this option should not be enabled for long periods of time as frequent write operations can damage the flash memory in the router, so after you download a troubleshoot file please change it back to RAM.
Also, your ping reboot option will not work with the current settings, as it will try pinging the 126.96.36.199 server every 120 minutes for 10 times, which comes out to the reboot of the device only after 20 hours of lost connection. If you do not have very strict data limits, it would be best to leave it on 5 min interval and interval count set to 1 or 2.