Try to signed your APK with version 1. Version 2 signed APK only
compatible from 7.0 so it will not working on below 7.0 devices or try
to generate v1 and v2 version APK.[ Note: When you generate signed apk for above 7.0 devices and below 7.0 devices then please check v1 and v2 both option. ]
Here is the reference link for more about V2 version and here is the link for overview.
Try this out and Your issue will resolved.