This is because of Vodafone Gigabox modem that drops packets in bridge mode. Nothing wrong with the TP Link device. Thanks for your help.

@Hank21 WAN 1 is static IP (PPPoE) and WAN 2 is dynamic. I plugged the cable of WAN 2 out and it is still the same problem.

I have setup a small network using ER605 using dual WAN. Ever since this setup, my WhatsApp calls on my cell phone (connected via WiFi) or on my laptop (connected via LAN) would get disconnected for...