So I am trying to see my 89-year old mother's Mac screen. I can initiate screen sharing via Messages, but she just cannot negotiate the dialog box to accept the connection.

What is the easiest way for me to take over her computer without her having to select something? I think I turned on screen sharing and I am an admin on her computer, so I think I can connect via VNC or something, but then I'd have to know her IP address, which can change. A long time ago, I used to use DDNS and was able to do this sort of thing, but that all broke for several years when her ISP changed her modem, and we can only do basic screen sharing lately, so I am starting over from scratch in a way.