Temporarily change your logging to a "permanent" storage, either an external USB stick or a directory that isn't /var/ or /tmp/. That way you have the log of what happened right before the reboot. You might want to consider logrotate if the reboots don't happen often enough before your logs get too large.
If you have enabled flow-offloading, I'd suggest disabling it for testing. There are threads in this forum talking about the connection tracking failing to prune its list, this might run you out of free memory as a side effect.
Edit: I overlooked that you were following the lede-17.01 branch, that obviously doesn't have flow-offload (which requires kernel >=4.14) yet.
Thanks for ideas!
I don't use the dls or WAN function. I converted the WAN port as a fifth LAN port.
The whole device is used as ethernet swith + wifi AP, router function is not used.
I'm acceptable linux user, but a newbie with LEDE, openwrt.
When I try to go in /var/log, it will go in /tmp/
How can I change the logs to be persistent?