Effective but inefficient solution is to reset the wifi radio signal (e.g. changing the channels, no need to reboot the entire router)
But for permanent solution, it is a tough one. I have changed the firmware to the third party - Merlin firmware, as suggested online. Does not seem to fix it.
So, per other site, I am changing the MTU from 1500 to 1492, RTS threshold from 2347 to 2304, beacon interval from 100 to 50.
Let's see how it goes.