Struggle - setting up internet connection

Need help!

I am just setting up a new network environment and need help,
struggling with the Internet connection ...

Using 4 Open-WRT devices, one CudyWR3000s should act as the main
router serving DHCP connected via ethernet to the other routers configured as AP. Was following the guide and within LAN all seems OK. Quick access to the static IPs assigned. DHCP served only from this main router.

Trouble starts with the internet connection.
I am using a Zyxel NR7302 with a strong mobile connection aprox. 500
mbit/s. I deactivated Firewall and activated dynamic PPPOE.
It serves DHCP and this is necessary for getting a connection to the main Open-WRT (Cudy) router.
For the router I applied the following settings:

  • Connecting the incoming ethernet cable to the routers WAN port
  • Use DHCP client as setting for the LAN connection (all other settings also PPPOE not working)
  • Fixed IP assigned and serving DHCP (this is the only DHCP server)

Generally it works, I see the WAN address assigned by the Zyxel and when
I try DSL speedtest I am getting the 500 Mbit/s, ping is arround 25.
But I do have a huge problem. The internet connection is completely unstable! Accessing internet pages is dramatically slow. Even accessing
the speedtest page is randomly not working.
WLAN (Router and APs serving same SSID) is constantly ON/OFF means not usable!!
I checked all settings multiple times but this situation is not acceptable!!

Anyone an idea what could be wrong here?
Anyone some knowledge for the Zyxel device maybe I missed something?

When replacing the main Cudy Open-WRT with my old landline connected router (they use the same static IP) all is stable the Open-WRT APs are all stable working.

Unless I'm missing something fundamental, this sounds like a really bad idea - never deactivate the firewall (and it's hard-required for NAT anyways).

But the first O-Wrt router connected via WAN port has firewall standard config means activated.
Nothing is connected via WLAN to that zyxcel even if possible. Just via LAN and the Zyxcel is on PPPoe mode.

Do you really think that is required?
I was following some forum discussions for that NR7302 and there all said deactivate firewall, so I followed .
Sure when my network will work I just can try to activate and see what happens