I'm using LuCI Master (git-18.081.35142-84b3ba4) / OpenWrt SNAPSHOT r6519-0f30f56 on TL-849N(BR), but sometimes the system is freezing or Wireless Stops. Then I need to restart the router.
How can I solve this issue?
Without logs, knowing what is going wrong is challenging to discover. It can be a challenge to capture logs when the router "crashes" as, by default, they are captured in volatile memory.
You might want to consider temporarily writing your logs to "permanent" memory (Not /tmp/ or /var/) or an external USB drive. Installing logrotate can help in making sure you don't fill up your "internal" flash.
How can I enable and change the log path?
For a "stock" system without syslog-ng or the like, there are details at https://openwrt.org/docs/guide-user/base-system/log.essentials and https://openwrt.org/docs/guide-user/base-system/system_configuration