You can also try this method.
Also, please be aware of how it works :
- xml file is a static list, provided with each ROM (and reset at each flash)
- what you use is a user database :
– it is recreated from xml file when hitting “Reset” in APN settings, or at first startup (when no user data exist)
– entries can be added to this database using touch interface, “content” command, or manually (it’s an SQLite3)
– this database can be reset automatically by system, for example when Google choose to change the APN storage method (it happens several times in past years)
So, I’ll recommend to write down added entries outside of phone storage.
Also, not easy but possible, you can retrieve the settings from a TWRP backup (it’s well-known that we all make backups before upgrading our devices ROM 
).