Red means the powerline signal is not good, too many interference in the electric wire between the two adapters. Did you try plug the two adapters in the same room to pair them again?

Thanks for your reply. I did try to plug the 2 adapters in the same room to pair them but to no avails.

Finally, this morning I unplug everything including the 2 adapters, routers, google mesh and all related power cables and waited for 1and half hours, luckily, after I plug all the devices, everything is back to normal including the adapters having 3 green lights.

As someone said, maybe it's related to the power of electricity elimination (not sure what it means).

Thanks for your help : )