Don’t know what you’re paying contractors, but this isn’t going to be cheap.
You’re probably looking at a cellular modem at each node or one cellular modem connected to a fiber optic or wireless LAN that connects all your nodes.
Regular WiFI isn’t going to cut it over those distances, you’d need professional wireless data radios and maybe a tower to mount antennas on if you go wireless. It would be easiest to build a fiber network between your nodes and then tie that into the internet cellularly or via satellite in a small data center on site.
And if your devices are mission critical and loss of connectivity could cause harm or damage, you need some redundancy built in in case you lose connectivity and that’s more money.
Be curious to see if anyone else has any recommendations, there could be a solution I’m not aware of available.