Neither Opal nor the Slate AX have a public IP. 4G likely have CG-NAT (=Carrier Grade NAT, so a NAT at the ISP so they can make multiple connections use the same IP). The Opal is behind the ISP router, so does not directly have a public IP.
As the instructions state, you can forward port 51830 on the ISP router to the Opal. That way the Slate AX can connect to the Opal using the public IP (of the ISP router) + that port.