MichaelS wrote

I cannot configure the AP as a receiver because of weak/no signal from the WLAN repeater in the basement.

You can not configure the AP as a »receiver« since the EAP does not support repeater mode. It's just an AP.

Your other questions regarding SSID roaming have been answered in this post already, but again in short form:

MichaelS wrote

Can I change the EAP 225 to fast roaming somewhere in his setup?

You would need Omada Controller for fast roaming. That's why I wrote that fast roaming will work with only EAPs.

Basically, fast roaming collects statistics about the APs in the same Extended Service Set (common SSID), so EAPs don't need to do a WiFi survey when clients are roaming. This is the only way to avoid delays when clients are changing APs. Without fast roaming, clients will have to perform a WiFi survey to find a better AP nearby when the signal from the current connected AP gets too weak. This WiFi survey and the reconnect is basically what causes the short delay in switching from one AP to another one.

But my question now: If automatic roaming doesn't work between AP in your 2nd floor and the basement since the WiFi signal level is still good, then this means that the EAP covers the whole area from 2nd floor to the basement already. Why would you want to use the FritzBOX WLAN on the basement if the EAP covers the whole area?

Personally, I would disable the FritzBOX WLAN and use only an EAP in the 1st floor which then probably covers the basement below as well as the 2nd floor above.