I received an email, sent about 1 hour ago telling me that "A new device has joined Archer AX50's network." I've seen this before if I actually acquired a new phone or other device, but I have not added anything.I was home at that time and no one else was in the house. I wanted to log onto the router to see if I could determine what the new device is, but when I tried to go to 192.168.1.1, my browser (Safari) warned me that the connection was not private. When I looked for details, it said that the certificate was not valid. Viewing the certificate, it is self-signed, with no issuer name, and it is for tplinkwifi.net. 

Does this sound legitimate?  Should I be worried?