If I understand you correctly…you have a wifi SSID called “upstairs” and then another wifi SSID called “downstairs”…then that means devices on the “upstairs” wifi will not be able to communicate with devices on the “downstairs” devices (and vice versa)…unless you have set up firewall rules on the router to allow communication between the 2 networks?
Ideally (or the way I have my home network set up anyway) you should have every 2.4Ghz only devices on a 2.4Ghz wifi SSID and then every 5Ghz capable devices on a 5Ghz SSID with only the WiFi channels being different between your upstairs and downstairs access points so they don’t interfere with each other.
So as an example of what I mean, when your on a device setting up which wifi you want it to join…you only want to see something like:
gpspad 2Ghz wifi
gpspad 5Ghz wifi
If a device sees both these SSID’s then it’s a 5Ghz capable device, so make it join the 5Ghz wifi…….if a device ONLY see’s the 2Ghz option, then it’s a 2Ghz only device, so make it join that one.
This saves a tonne of headaches when devices try to be clever and move/roam between access points that broadcast both frequencies under the same SSID.