Constant popup browser window about joining my own wifi network
Since mid-summer 2018, I have been constantly interrupted by a popup window looking like a browser window, with a title Join "[my name] Wifi Network" - although I am already on the wifi network. It's actually the Apple website - there are menu dropdowns about all their products. Oddly, this window insists on being in front - no other window can go on top of it. And I can't get internet access until I close this popup. It's a nuisance, one of those things you might just put up with, if it weren't like every 20 minutes. Why did this start out of nowhere, and how can I stop it?
Here's what the popup looks like:
I'm on an iMac, running OS X 10.10.5 (I note the dropdown in this posting space doesn't provide that OS...), with Safari as my default browser. Any help appreciated.
iMac, iOS 10.3.3