wpa3 connectivity
I just updated my router from wpa2 to wpa3, and now my imac (macos 10.12.6), and now I cannot log in to the wi-fi. The pop-up box says the router needs a "wpa2" password.
iMac Line (2012 and Later)
That macOS Sierra and your older iMac are unable to make WPA3 handshakes with new router hardware. That router hardware should be configured to provide WPA2/WPA3 compatibility rather than just WPA3, and I would be surprised if a router vendor could transform an older WPA2 router to WPA3 functionality with just a firmware update.