RT-AC3200 - Connection: Ethernet link down. / Connection: ISP's DHCP did not function properly.

Since I started using RT-AC3200, my internet connection intermittently drops for 10s-120s several times a day.
Tested firmwares both Merlin 378.55.0 and ASUS 3.0.0.4.378.7838 but they both exhibit the same problem.
Changing cables (CAT5e/CAT5e STP/CAT6) has no improvement at all.
I suspected if it was something to do with Dual WAN but turning it off doesn't help at all.

While I'm testing various scenarios, I found some interesting facts.

When the connection drops with static IP WAN, I...

RT-AC3200 - Connection: Ethernet link down. / Connection: ISP's DHCP did not function properly.