The step you are missing is putting data in NewDF. I do not know what kind of debt you a dealing with so I will assume personal debt and I will put values from 0 to 500,000 in NewDF and set bcour, gvex, and tradeop to 1. You will have to adjust all of that to use appropriate numbers.
NewDF <- data.frame(debt = seq(0, 500000, 50000), bcour = 1, gvex = 1, tradeop = 1)
NewDF$PredInvest <- predict(fe_model, newdata = NewDF)
You should then be able to plot the result.