Hot water will not work to remove grout. Unfortunately, concrete and grout are what we use to make larger sewer lines. Since grout sets up under water, you may have to get a licensed plumber with powerful sewer and drain cleaning technology to try and clear the line. If the line is plastic, you may be able to break up the grout and push it down to a larger line. But if your pipes are steel, then it may be impossible to get the grout out of the line, unless it is a very thin layer. If it is a lot of grout, it may break loose and go down and clog another line. Sometimes it's necessary to break up floor drains and other lines. Unfortunately there is no easy solution for this drain problem. Try snaking the line after removing the trap for the bath tub. If that doesn't work, a few feet of the line might need to be replaced.