Properly integration of Zyxel AX7501-B0 and NWA210AX

I've the two above mentioned Zyxel devices and several times per day my internet connection (fibre) is very slow, so that applications like video-calls or web-radio are interrupted. I want to ensure that I've a good / valid setup chosen and that the issue for the problems is not related to my configuration. Below the key aspects of my setup:
  • The AX7501-B0 is connected to the fibre and is located in my cellar in the ground floor. Here I've a WLAN GROUND_FLOOR created.
  • The AX7501-B0 is connected to the NWA210AX via a 1 GB LAN cable. The NWA210AX is located in the first floor and creates a second WLAN FIRST_FLOOR.
  • This setup works generally and I have from a signal perspective a small overlap between the two different WLAN networks.
  • The overall IP range for the AX7501-B0 is 255 (devices) and both WLAN networks are able to hand-out new IPs of the same range. There exists therefore a small risk that the same IP might be given to two devices at the same time. 
For me is not clear how I could configure with the two devices only one network like WLAN FAMILY and the NWA210AX would just repeat the WLAN (over the LAN capable).

Any thoughts are highly welcome.

Consumer Product Help Center