How to connect two devices at home using 5G Gateway TMO-G4SE

I have 5G Gateway TMO-G4SE at home and I have a number of devices such as cell phones and computers connected to the Internet from home.

I noticed that I can't connect to any of the computers from other devices (computers for example), even when they are both at home, and both connected to the Internet using the same gateway. The connection times out. Either ping or Telnet would time out.

How can I connect to a computer setup at home from another computer?