How to fix Heatmap being masked by other layers in Leaflet

Question

I am trying to use Leaflet in R and I want my map to have two layers: one for a heatmap and one for polygons. The problem is that polygons always stay on top of the heatmap whereas I want the heatmap to appear on top of the polygons. Below is an example code:

library(leaflet)
library(leaflet.extras)

leaflet() %>%
  addPolygons(lng = c(10, 20, 20, 10), lat = c(10, 10, 20, 20), fillOpacity = 0.75, group = "L1") %>%
  addHeatmap(lng = c(14, 15, 15, 16), lat = c(10, 10, 12, 10), group = "L2") %>%
  addLayersControl(
    overlayGroups = c( "L1", "L2"),
    options = layersControlOptions(collapsed = T),
    position = "bottomleft"
  )

Someone gave a partial solution to this here: Controlling the Z index of a Leaflet heatmap in R

However I couldn't figure it out how to solve the issue using that answer. Any helps would be appreciated.