To use fonts in iOS, you need to download a font based on the FULL NAME (PostScript Name) font, which sometimes (and usually) differs from the actual FILE NAME file name.
Imagine that you are renaming the font file "Arial-regular.ttf" to be "Foo.ttf". The font contained within the font file that you just renamed is still "Arial-regular."
There are some good programmatic ways to get the font name already in this thread, but I have a different approach using the command line.
If you are on Mac or Linux, just run this script from the command line in the directory where you have your own custom fonts (uses the fc-scan utility from fontconfig , which is already installed, but if you cannot install it via homebrew: brew install fontconfig ):
for file in "$arg"*.{ttf,otf}; do fc-scan --format "%{postscriptname}\n" $file; done
Here is a screenshot of the above command running in my ~/Library/Fonts directory:

The script above will go through all the .ttf and .otf in the current directory and then print a PostScript Name for each font that you can use to refer to the font file in Xcode or elsewhere.
If you need additional information (PostScriptName, Filename) and some color coding, you can run this alternative script:
for file in "$arg"*.{ttf,otf}; do postscriptname=$(fc-scan --format "%{postscriptname}\n" $file); printf "\033[36m PostScript Name:\033[0m %s \e[90m(%s)\033[0m\n" "$postscriptname" "$file"; done

This is slightly faster than copying the code inside the AppDelegate.m file to print the names every time you want to add a new font file, which is a popular method, and also faster than opening a font in FontBook to check the PostScript name.
USEFUL TIP: If you use the above script alias in your terminal, so all you need to do is enter a single command to get all PostScript font names for all the files in the current one (my function is called fontnames , so all I need to do , this is the type of fontnames on the terminal inside the directory with the fonts in it, and the PostScript names will be printed automatically, then you will save time in your development workflow and this convenient script is ready to use when you need it.
Hope this helps!