After a morning of waiting and trying to compile and install R from source code with only limited success, I found out I missed the correct link which allowed me to install the latest R-3.4.4 within seconds.
Here is the link: https://stat.ethz.ch/CRAN/bin/linux/ubuntu.
In principle, one needs just three steps:
Step 1: Add source
Add the following line to /etc/apt/sources.list
deb https://stat.ethz.ch/CRAN/bin/linux/ubuntu xenial/
stat.ethz.ch/CRAN with your favorite CRAN mirror)
Step 2: Add the CRAN key
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9
Step 3: Install!
sudo apt-get install r-base r-base-dev
r-base-dev is only needed by package maintainers)
In case you need to install the latest R in Linux MINT or Ubuntu, I would suggest not to start from the source code, since it requires a lot of dependencies and the compiling is very lengthy. This was not the case a few years ago. Now it makes much more sense to install from the CRAN source.