I'm building a world, which takes place about 300 years in the future after an event in the near future, in which giant elemental monsters started to roam around the earth. These creatures are an average of 30 feet tall and have shapes of normal animals. They were resistant to conventional weapons and took the world by storm. After fighting humans and amongst themselves, these creatures have settled into large territories. The territories are large, like the size of New England region in the United States.
Humanity has been knocked back a bit to about industrial revolution technology and formed into city states inside the territory of the giant creatures. Humanity and these giant elemental creatures have learned to live with each other.
The giant creatures have learned to tolerate those within its territory but patrols their borders strictly. Any infrastructure between territories don't last long, except basic roads.
My question is how would the city states communicate with each other, if infrastructure can't cross the borders?
I was thinking something like radio. Would that infrastructure last 300 years or easy enough for industrial level people to build if they had the means?