Just ignore the error and select "continue to the website."
There is no way to have a valid SSL certificate because it's a private IP address, and it would become invalid the moment you changed the IP address on the router to something else anyway. You will notice this error any time you have a router that allows connection via SSL/HTTPS.
Even without the protection of a SSL certificate connecting via SSL is a good idea because it prevents the majority of the traffic from being in clear text. You are still vulnerable to a man-in-the-middle attack without a valid certificate, but for it to work the man-in-the-middle would have to be there at the start of the connection -- if he came in late it wouldn't be possible.