Why don't you install a VISIBLE electric fence? I am working with two big dogs who freeze in their steps when they are outside after the owners installed one of those abusive invisible devices. They won't move a step and have a devastated look in their faces because they were severely pushed just by being outside; they don't know what hit them and why and changed from normal dogs into basket cases.
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015