The name is properly written Ficus benjamina 'Rianne.'
it was outdoors all summer but now it so cold under 0° so I've put it
inside...

Correct. I assume you are in Europe.

the temperature is around 20 (70 F) and it is near a window so it gets as
much light as i'm available to give it to HIM!
but its leaves turn yellow

This is to be expected with some Ficus cultivars when you bring them indoors.
Try to keep it a little cooler, if possible. Water less often during the
winter. Don't water until it starts to get dry. Don't feed it until it starts
to grow. Turn it about once a week so it is evenly lit. If you can put it under
a fluorescent light for 18 hours a day, it may do better than in a window.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)