You are correct. Most fridge thermostats can't go much higher than the mid 40s. Most lagers need to be fermented in the low to mid 50s. An external temperature controller will achieve this. Once the beer has fermented, the fridge thermometer will work fine for the "lagering" phase.
Also, a fridge themostat will allow much larger temperature swings than a controller will. An good controller will keep the temperature within one degree of the desired point.