@Yeye
Can you let me know a bit more about your network topology. For instance, where is your router? Also, you said you had an extender upstairs. Is it a Meraki Access Point or a 3rd party WiFi is Extender?
Normally all access points would be plugged into a router, or switch. The Meraki WiFi meshing will kick in when the link to an access point goes down.
If you cannot run a cable to a Meraki access point, it will connect to the nearest other Meraki access point in mesh mode. Throughout will be limited because the Meraki access points do not have a dedicated to mesh radio. Performance will be best the nearest access point is in line of site. The more building components in the way, the lower performance will be.
Dave Anderson