After changing batteries in both the outside sensor and the inside clock I no longer receive a reading for outside temp.

Have you changed the batteries in the outdoor temperature sensor?

Place your remote temperature sensor and the inside unit together for about an hour to see if they both read the same or if there is a large difference in the temperatures.

Dashes on the outside temperature reading may indicate that the inside unit is not receiving a signal from the remote. Change the batteries on the remote or change the location of the inside and/or outside units so that they can receive a strong signal between each other.

Your smartphone temperature may be coming from a location that is not near your home. They are often from a weather service. Where is your local temperature sensor located? Is it in the sun? Is it on the south side of the house, or another location where the sun or an air conditioner or other heat source may affect the accuracy of the reading? Try to locate your outside sensor in a shaded location with plenty of ventilation, and away from heat sources.

I had the same problem, Since the outside temp wouldn't read, I figured it was the outside unit batteries. Went through half a dozen AAA batteries and wouldn't read. It was the indoor unit. Had the same bats I installed 3 years ago. DUH! Replaced the AAA, and all was well. When the inside batteries die out, it has just enough juice to read indoor temp, but not enought to sense exterior signal.

I read the directions on the back and it is working 100%!

Simple, the instructions state that when changing the batteries on the outside temp. sensor, you must also change the batteries on the main clock unit. I had same problem when I changed the sensor batteries until I pulled the batteries from the clock and it reset fine. ( Sensor batteries first, then clock batteries. ) The clock will reset to atomic time overnight so don't worry if it doesn't read correctly at first.

I keep the outside unit of our in/out set under a coffee can shaded under a bench; one could argue the aesthetics but shielding it from wind and sun is mandatory otherwise one can get record temps pretty often.
The outside units generally transmit to the receivers only every few minutes to keep battery consumption down so moving them won't necessary gie you an immediate change.

I've had a similar problem with another model, an Oregon Scientific Model IWA-80055 Wireless Thermo Clock, in which when I replaced the batteries in the remote sensor, the clock would not read the temperature from the remote sensor.
I found it fairly consistent to solve this by taking a battery out of place in the clock and putting it back in.  Unfortunately this also messed up the clock's time and date.  This is also in the instructions catalogue for the device, check and see if yours gives a similar solution.

Try changing the battereis in the CLOCK. When the batteries in the clock start getting low, the clock does not pick up the outside transmitter.