On Thu, 11 Oct 2018 16:17:54 -0400, Tekkie® wrote

I agree Tapcons. You might be able to get in aluminum or plastic to prevent
rust.

Tapcons?

How to Fasten a Sign to Concrete Using Tapcons
https://www.concretefasteners.com/di...e-with-tapcons

That seems simple enough!
I buy the Tapcon bolts at Home Depot & drill one into each sign corner.