Now I'm having problems with the new mesh WiFi. Every 2 hours or so the mesh WiFi would keep on disconnecting with the router (no Internet connection, according to iOS app) and I have to turn off the whole Maxis router and VPN router to get back online.

My setup is Maxis Fibre 300 Mbps

1. Maxis router default configuration 192.168.1.254

The Maxis router connects to Asus RT-AC68W

2. I have a Asus RT-AC68W, setup as a ExpressVPN router, on a different network segment. 192.168.21.1. Usually I set the WAN to automatic DHCP configuration and before this no problems at all. Restart once in a while, but not at often since I installed the TP-Link Deco M9 Plus. I tried to set WAN to static configuration as below

IP address - 192.168.1.75
Subnet mask - 255.255.255.0
Default gateway - 192.168.1.254
Primary DNS - 192.168.1.254

The Asus router connects with TP-Link Deco M9 Plus

3. TP-Link Deco M9 Plus, 3 set. I set as router mode.

I tried to set automatic WAN via IPv4. But still get disconnecting every 2 hours or so. And I have to restart the whole system. 

I tried also manual WAN as below

IP address - 192.168.21.75
Subnet mask - 255.255.255.0
Default gateway - 192.168.21.1
Primary DNS - 192.168.21.1

The LAN IP is 
192.168.0.1

Any ideas what causing this problem?