Could not find function in R even though it is installed (coefplot and visreg)

I am trying to use the functions "coefplot" and "visreg", but for both it always shows this:

install.packages("coefplot")

> Installing package into ‘C:/Users/Documents/R/win-library/3.5’
(as ‘lib’ is unspecified)
trying URL 'https://cran.rstudio.com/bin/windows/contrib/3.5/coefplot_1.2.6.zip'
Content type 'application/zip' length 139328 bytes (136 KB)
downloaded 136 KB
package ‘coefplot’ successfully unpacked and MD5 sums checked
The downloaded binary packages are in
C:\Users\AppData\Local\Temp\RtmpaGsWOZ\downloaded_packages
> coefplot(zib_model1)
Error in coefplot(zib_model1) : could not find function "coefplot"
> install.packages("visreg")
Installing package into ‘C:/Users/Documents/R/win-library/3.5’
(as ‘lib’ is unspecified)
trying URL 'https://cran.rstudio.com/bin/windows/contrib/3.5/visreg_2.5-0.zip'
Content type 'application/zip' length 284709 bytes (278 KB)
downloaded 278 KB
package ‘visreg’ successfully unpacked and MD5 sums checked
The downloaded binary packages are in
C:\Users\AppData\Local\Temp\RtmpaGsWOZ\downloaded_packages
> visreg(zib_model1, "pop")
Error in visreg(zib_model1, "pop") : could not find function "visreg"

I am using this R version:

R version 3.5.1 (2018-07-02) -- "Feather Spray"
Platform: x86_64-w64-mingw32/x64 (64-bit)

I already uninstalled and reinstalled both R and R Studio several times. I tried installing the packages in Administrator mode both in R and in R Studio, like several answers to other similar questions on this website suggested.

Could anyone suggest what I should do next?