Got a Singtel Wifi Gigabit Router AC1900 (Arcadyan) and notice that my wifi was unstable as the dual-band router was constantly balancing between the 2.4 GHz and 5 GHz bandwidth.

The solution for me was to disable the band steering and the dual-band wifi become stable again. Although the auto switching of band width is a good feature, the technology was not perfect yet. So, I will stick to manual where I decide how many device should be connect to the 2.4 GHz or 5 GHz.