setting up ax3000 mesh as AP. need help

[Rainmanone]

Hi.
I've been spending days with this, and Xiaomi support just recites the manual when asked.

I have a Segemcom fiber modem I'm using as my main router , this is the DHCP server.
it is connected with wires to every room in my house.

I disabled it's own wifi and trying to set up the AX3000 Mesh system with this configuration :
1. Main WAN port node connected with a cable to a LAN port in the Segemcom.
2. Second node WAN port , connected to a LAN port of the Segemcom router
3. Mesh DHCO is disabled .

before disabling the DHCP in the mesh, I get about 960 Mbos near the secind node, but about 150 Mbps near the main node.

when dhcp is disabled , i have no more access to the web interface of the main node

also , my phone will not connect to the wifi network , it says it can't get an IP address.

what am i doing wrong ?

can someone supply step to step instruction on how to set it up ?

thanks