Solution with cascaded routers

[chrossi2000]

Hi everybody,
I have two routers in my home, one is to connect to the cable internet and another one is a fritz box in the office to create the wifi.
My Deye inverter is connected to this wifi. As long as I have Home Assistant running with a connection to this router, everything works fine.
However, I want to connect the home assistant to the other router, which is "in front" of the fritz box.

I tried to open the port 8899 for the Deye inverter on the fritz box, but the home assistant integration now doesn't get any data.

Is someone running a similar setup or has any idea what I need to do?

Thank you!