I would like to use custom font in my app, but I keep getting error
[RCTLog][tid:0x78f70450][RCTConvert.m:581]>Unrecognized font family \'Roboto\'
That's because iOS does not have the Roboto font available by default.
Exactly. To make it available you have to include it in your project from Xcode. Just follow the steps that this answer provides, it's pretty straightforward.
Once you have done this, then you should have no problem using it from JS.