I haven't tried this exact procedure on a VM or anything, so be aware of that. Hopefully this also fixes the mucking about with /usr/local/bin/R alluded to in the question. Using the command line I'd attempt the following:
Show available versions provided by the r-base package
apt-cache showpkg r-base
This emits a bunch of text to your console. The important bit should be at the end about versions provided. You'll need to choose one of those. For me the only 3.4 listed is 3.4.4-1ubuntu1. Write that down or copy-pasta it somewhere.
Purge existing R package
Unless you want to attempt to manage two R installs on the same system, I'd blow away the existing R installation.
sudo apt-cache purge r-base
Then clean up any packages that are no longer needed
sudo apt-get autoremove
Install package of specified version
Heat up that copy-pasta we made earlier and stuff that into the package version specification for apt-get
sudo apt-get install r-base=3.4.4-1ubuntu1
Mark the package to not be upgraded
sudo apt-mark hold r-base