With the exception of the occasional boat trip from the villages of Naknek or King Salmon, Brooks Camp (this part of Katmai National Park) is only accessed by small float plane. See here for more information: nps.gov/katm/planyourvisit/directions.htm
You should expect to be totally without cellular signal while at Brooks Camp.
Most hiking and walking trails start at the Brooks Camp Visitor Center, which is very close to where float planes land on the shores of Naknek Lake.