What temperature is the "warm" setting on a conventional oven?
The collagen in the ribs needs to get to a temperature of about 160 F to start breaking down into water and gelatin.  If your oven can be set as low as 175 or 180 F on a normal bake or convection bake setting that is probably a better alternative unless you have access to an oven thermometer and can measure what temperature the warm setting on your oven attains.