There is an application called GarageBand you can use to add M4R ringtones to your iPhone. The application can be found on the app store. Below is the icon of the application. A step by step guide will be provided on how to add the ringtones to your iPhone.
Step 1: After installing and launching the application, tap the "+" button and then tap the "Create New Song" option.
Step 2: Select the "Audio Recorder" option when the application ask you to choose the recording type.
Step 3: Tap the multitrack button to move the track view.
Step 4: Tap the loop icon and select the "Music" option. If the desires file is stored in the Files section, select the "Audio Files" option.
Step 5: After finding the desired ringtone, drag it to the track and trim it as you like. Please keep in mind that the maximum length of a ringtone is 30 seconds.
Step 6: Tap the flipped triangle button (or the file icon on iPad) and go to "My Songs".
Step 7: Press and hold the track until it wiggles. Click the share option and export it.
Step 8: Choose the format as "Ringtone" and rename it as you like.
Step 9: Go to the Sound & Haptics setting of your phone and see if the ringtone has been added.