I bought a new Asus RT-AC1200G+ to replace my RT-AC51U for better signal and Gigabit Ethernet. It’s much faster to copy files to/from QNAP TS-119PII now. But I notice it drops internet connection several times a day, I am sure there is nothing with my tethering device as described in “Android USB Tethering Drops and Data Usage Crashes, try Reset“.
RT-AC1200G+不定期重啟 has discuss the same issue. It seems related to bugs in current firmware (220.127.116.11.380.3310). I have to download version 18.104.22.168.380.4089 released on 2016/11/14 from RT-AC1200G+ Driver & Tools and upload via [Administration] → [Firmware Upgrade] tab. It will begin to flash and reboot automatically.
My RT-AC51U works without dropping for 3 days after the upgrade but still generate some warning in system log. I will keep an eye on it and update this post.