[crash] problem with OpenWrt + luci

i have a:

Model	TP-Link TL-WR841N v13
Firmware Version	OpenWrt SNAPSHOT r7115-8796680277 / LuCI Master (git-17.194.28316-2224714)
Kernel Version	4.14.48

this router is at my work and merge 2 separate subnet (1 online [sub1] and 1 offline [sub2]) and host a vpn for access the offline vpn

i noticed that every day at 8~10 pm the router stop work (vpn stop, can't access from sub1 to sub2, can't access luci page, can't access ssh)
i think this happen when the server in sub2 is doing the closing routine of the day (i don't know what it does in that time, probably send the data to another server in the intranet) is possible that it fucks the router in the process?
what i can do for find the cause and solve it?

i tried to reboot the router as soon i noticed the problem but after plug back the power, i found it still stucked

i'm using a cron for reboot but i don't think this help

40 20 * * * sleep 70 && touch /etc/banner && reboot
30 12 * * * sleep 70 && touch /etc/banner && reboot