The word dimension has a very specific technical meaning in mathematics and physics. It’s basically a number needed to specify something. For example the surface of a sheet of paper is two-dimensional because you can specify a point on the sheet of paper using the Cartesian coordinate system. You can specify the time and place of an event in the universe by using three cartesian coordinates for space and another number for time. This makes space-time four-dimensional and time is one of those dimensions. So answer time is a dimension. But this isn’t the science-fictiony thing some people expect. It just says you can specify a time using a number. That’s it.
But knowing that it takes four numbers to specify an event doesn’t change anything. By definition of past, present and future, the past and future don’t exist simultaneously with the present. However, it does give you an interesting mental picture. If space-time is four-dimensional, and a sheet of paper is two-dimensional, you could imagine the universe laid out as a diagram on a(n imaginary) four-dimensional sheet of paper. You can sort of imagine the past, present and future existing simultaneously in such a picture. What’s more, such a picture is quite useful sometimes when doing physics. But imagining a picture doesn’t literally make the past simultaneous with the future.

Answer by Dan Poponi to the question If time is a dimension, is it correct to say that the past, present, and future exist simultaneously?