Network setup with one static IP

I have a networking question that I know has an answer, but I cannot seem to understand solutions. That or I don’t know what to Google.

I have a public static IP line from Spectrum Business that was installed to monitor a HVAC system. The computer used for HVAC control is set up and online, but that seems to be the only device I can add to the network. The IP address assigned to that computer is the public IP Spectrum assigned to us. I’m trying to figure out a way to get the Ubee modem/router to assign IPs with DHCP, but when I connect to the router’s Wi-Fi, I can’t even access the router’s UI.

I’ve been searching Google and YouTube for tutorials on how to set up a static IP network, or even one with additional DCHP control on top of one static IP, but the two don’t seem to mix. Any ideas?