TMAP - mask everything outside of borders in plot

I have the below code

Map <- tm_shape(DotsToPlot)+
  tm_dots(col = "red", size = 0.2)+
  tm_shape(AreaBoundaries)+ 
  tm_borders(
    col = "black",
    lwd = 1,
    lty = "solid",
    alpha = NA,
    zindex = NA,
    group = NA)+ 
  tm_facets(by = "Name")

which creates multiple maps with area boundaries and dots overlaying to signify areas of interest. The problem is that faceting in to the six different areas works fine but the dots are not restricted to the border of each facet. Could anyone advise please how I can correct this so that only dots that correspond to each area are shown on each of the 6 maps OR that I can add a mask that creates a white layer over the top of any areas outside of the border so dots outside of each border are masked?

Thank you.