I had router
on first floor and landline on ground floor in a diagonal but same side room a couple of years ago (Not able to describe exactly, but basically the room was not right below the one where router was and a bit away, more like an adjacent apartment). Airtel did it for me, and they changed it back multiple times whenever I asked them to do a setup change. Not sure if the experiences are different than I had in Chennai.
If I remember correctly, they had brought the line inside the first floor room and setup the router and then had taken another wire out to the ground floor room to provide the landline
Now its the same room, once I got Hathway as my alternate connection.