install.packages("drat", repos="https://cran.rstudio.com")
drat:::addRepo("dmlc")
install.packages("mxnet")

Package which is only available in source form, and may need compilation of C/C++/Fortran: ‘mxnet’
These will not be installed

library(mxnet)
Error: package ‘mxnet’ is not installed for 'arch = i386'