Hi,
The plotting in Prime is one of two options Plots | Traces | Insert Plot or the Math | Regions | Chart Component.
In Chart component grids are possible. In "Insert Plot" grids are not defined by Prime.
The converter from Mathcad 15 to Prime always uses the "Insert Plot" not the chart component hence no grid.
Two options to proceed are possible.
a) Change to a Chart Component which has grids.
b) Research the community where LucMeekes and Werner_E have made grids possible in the "Insert Plot" plots
Enclosed is Werner's