Before calling service technician, open following link for Rheem troubleshooting resources.http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Sometimes people attach tankless to tank-type heater to preheat incoming cold water.
Additional tanks are also used to store solar heat, and some installations have tank-type because tankless is unreliable. Service technician can sort out the function correctly.http://waterheatertimer.org/Tempering-tank.html
Owner should read tankless manual thoroughly, especially troubleshoot and maintenance sections. Tankless require yearly maintenance.
Since tankless are expensive to repair and replace, you might be able to fire up the tank-type water heater and provide hot water without much cost.
You can also install tank-type gas water heater yourself. Lowes sells 40 gallon for $317.00, and then you offer 10% less.