Frozen sewer lines are a mess.
Fortunately, they don’t happen as often as frozen water lines. It is very hard to use a torch on 4″ ABS pipe without damaging it, and the volume of ice requires LOTS of heat to unthaw.
My least messy, least dangerous, but slow approach was to attach heat tape to the entire length of the frozen line and then cover that with insulation bats. It still took 24 hours to unthaw.
Especially if the line is only froze in one or two places the heat from a hair dryer would probably work well.
Depending on the circumstances, it might also be possible to thread a water hose into the cleanout and use the water pressure to break/thaw through the blocked area.