Is there a way to get kernel logs from a previous boot?
I have tried using journalctl but logs are only stored for the current boot.
Is there anything I have to reconfigure to keep older logs?
Are you asking about an Ubuntu Core install ?
If so, you can create the /var/log/journal directory and it will start writing logs to disk, so they survive a reboot and show up in journalctl.
Thanks for your answer @ogra! yes, small detail 
, I’m running a Ubuntu Core install. I suppose log rotation and persistency is all managed by journalctl too?