To export a complete log you'll have to navigate to your RUT955 WebUI System > Administration > Troubleshoot.
Click on Troubleshoot file "Download" button and every log you need will be archived and downloaded for you.
When so called "trouble_backup_Teltonika-RUT955.com-20xx-xx-xx.tar.gz" package is downloaded, inside you'll find a folder with 4 log files: all_eventsdb.log, network_eventsdb.log, system_eventsdb.log and system.log.
As it happens system.log file is stored in RAM memory and therefore it resets every time router restarts.
Let me know if there's anything else I can help you with.