You will need the program ASUS Firmware Restoration to be able to install the Stock Firmware from Asus.
You can get the software and firmware from here:
https://www.asus.com/us/Networking/RTAC56U/HelpDesk_Download/
You can use this guide to help you (it is for the 68U, but should be the exact...
Did John's fork solve your problem? I'll give it a try if I can't find a solution with the current firmware I am using, as flashing is always a hassle.
WISP is my ISP
I mention WiFi channels to make it clear the lag is not from too many devices connected to one channel. I have two routers and 4 wireless access points (two 2.4 GHz and two 5 GHz radios), all on different WiFi channels so that they do not interfere.
The POE is the modem that my...