The DaTuner program I used was inexpensive and easy to use until it
was sold and upgraded.  See

https://prometheusinteractive.com/2018/11/06/datuner-help-and-overview/ 

Some updates of Android OS itself are not compatible with DaTuner
programs.  Presently there are a dozen or so Android tuning programs
to consider on Google Play Store: https://play.google.com/store/apps 

My Marshall & Wendell upright is kept at a lower non-standard pitch
(Hz) because the strings are old -- but serviceable.  The 90+ year-old
strings are likely to break if brought up to A=440 Hz standard pitch.

Are there users who have found a piano tuning app that allows for
lowering the reference frequency of the tuning program?

In this very dry desert climate the piano seems to like being somewhat
flat.  Programs I have sampled do not readily adjust to a lower pitch.

Bill Chapman - today it's 73°F with 17% RH
Palm Springs, California