You were cited for "failing to obey a traffic sign" that stated "Road Closed to Thru Traffic".
It didn't say "Road Closed to Thru Traffic Unless There Are No Detour Signs". So while it would be nice if they had posted some detour signs to show you which way to go, I don't think you can reasonably argue that you were not in violation due to the fact that they did not post any detour signs.
You can check the Federal Manual for Uniform Traffic Control Devices HERE and then check the Wisconsin Supplement to that manual HERE but I highly doubt that you'll find anything there that supports you claim that you can disobay that sign if detour signs were not posted.

