I just put the files on a thumb drive and connect it to my phone and copy them over to my internal storage since that's where the folders are. The path is something like emulated/0. In that "0" location there will be a "Ringtones" folder and a "Notifications" folder. If you don't have a thumb drive adapter or a 2 sided one that plugs right into the phone, just email yourself the files, then save them to the notifications folder in the path I mentioned above. If you need more info, I can make a video of the process.
You might actually need to make the file a shorter length for it to work in the notifications folder. If so, I use Mp3 Cutter specifically for the purpose of making ringtones and/or notification sounds.
https://play.google.com/store/apps/details?id=mp3.cutter.ringtone.maker.trimmer