Hello,
You are writing that the WiFi.RSSI() does return a value in dBm.

https://www.arduino.cc/en/Reference/WiFiRSSI

Returns

long : the current RSSI /Received Signal Strength in dBm

According to the specification the RSSI value is only a relative value, comparable to a dBm value but not the same. The relation between the dBm values for a specific RSSI do change between different vendors.

Please could you clarify this?
Does the Arduino API really return a dBm value for the WiFi.RSSI() call?

Thanks in advance.

Bye
Rainer