finally I found two ways to run R on endless:
install either jamovi or JASP or both. Both bring their own version of R with them.
use either of the following commands:
flatpak run --filesystem=host --command=R org.jamovi.jamovi
flatpak run --filesystem=host --command=R org.jaspstats.JASP
optionally followed by arguments to R
you may create an alias for that, e.g.:
alias R='flatpak run --filesystem=host --command=R org.jamovi.jamovi'
I don’t know if
--filesystem=host is good or you would better use a different filesystem option. I also do not yet know, if graphics output works or how to display it …