The only access I have to the ISP's wi-fi router is changing the SSID/PWD and check the devices connected. Roku stick still cannot connect of course, but still working on that. Updating to ASUSWRT-Merlin 388.1, from version 386.7.2, seemingly resolved most of wi-fi devices connectivity issues.
My wi-fi gear's WAN port have been connected into the ISP's ethernet LAN port, where this setup worked just fine for couple of day after getting the fiber connection.
