You are currently logged in from another device. There can be only one device logged in at a time. I

Yes, the Orbi web server allows connection from only one IP address. When it detects a connection attempt from a different IP address, this warning pops up.  There can be simultaneous connections from different browsers as long as they are on the same computer.  (I just did it as a test.)

How is this Mac connected to the router (wired or WiFi)?

(Recent software updates created all sorts of confusion when the default became to randomize the MAC address when connecting to WiFi rather than use the actual hardware MAC address.)