The city continues to get things half right on campaign signs. They must be on private property. If not, they are illegal.
However, as long as they are on private property, it is an unconstitutional prohibition for them to say you can only put them up 30 days before an election.
As a private property owner, I can make political statements via signs anytime I want. Just ask the US Supreme Court. I quoted their decisions to the city in the past and they agreed. Now they don't. Get your act together folks, and get rid of the illegal ones.