Ringtone set from a file reverts to default after reboot
Apps such as Tasker can definitely set the ringtone after device boot. 


Profile: Event &rarr; System &rarr; Device Boot
Task: Audio &rarr; Default Ringtone:


Type: Ringer
Sound: tap lens icon &rarr; choose File Manger app -> select your sound file.



File Manager app should be such app which allow a sound file to be selected as a ringtone. I'm aware of Solid File Explorer and ES File Explorer providing such feature.

If you don't get the option to use the File Manager app, then clear the defaults of Media Server from Settings &rarr; Apps &rarr; All apps (or three dots line &rarr; Show system) &rarr; Media Server &rarr; Open by default &rarr; Clear defaults.