Brisket is cooked when the connective tissue is properly rendered. This happens sometime after 195ºF (90.5ºC). But you don't use temperature to tell when your brisket is done, you use texture. Use temperature to know when you should start checking (e.g. at 195ºF).
There are many different ways you can do this. Depending on who you ask they will give you their favorite technique. Some of the popular methods are:
I personally use the first method. Using it properly is really about experiencing it. It can be hard to grasp until you actually try it. For my first brisket I was unhappy with checking for "jiggle" since it didn't seem like a very precise way of cooking. After I actually observed the jiggle in my brisket did it become apparent that its actually quite an effective way of telling when it is done.
As per the 54ºC...while technically cooked, it will be a terribly tough brisket. I wouldn't suggest it. Unlike a steak, brisket benefits from rendering all the connective tissue.