Unknown router connecting to my gateway for first time

Unknown Technicolor router connected to my gateway for the first time. I paused connection indefinitely. Have never had this issue. I keep track of every device on my network and get notifications whenever someone connects. We have one router it’s a tp link and it’s still connected. This new technicolor router has a MAC address but no IP address (not sure if that’s cause I paused the connection).
Would like to know how people are getting on my network no one could possibly guess my combination of characters in my password.