#[Japt](https://github.com/ETHproductions/japt), 61 bytes

I was working on this earlier but gave up when I read further down the post and realised conversion had to work both ways. Thankfully [obarakon](https://codegolf.stackexchange.com/users/61613/obarakon) convinced me to take it up again and helped me save 2 bytes as well.

Takes input as a string for both operations and returns a string for both as well.

    V="OIREASGTBP"UÉ?¡VgXÃe"%v%v"_i1'NÃe"%V%V"_i1'UÃ:Ur"N|U" £VaX

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##Try it
- [Numbers -> Letters](http://ethproductions.github.io/japt/?v=1.4.5&code=Vj0iT0lSRUFTR1RCUCJVyT+hVmdYw2UiJXYldiJfaTEnTsNlIiVWJVYiX2kxJ1XDOlVyIk58VSIgo1ZhWA==&input=IjMxNDE1OTIiCg==)
- [Letters -> Numbers](http://ethproductions.github.io/japt/?v=1.4.5&code=Vj0iT0lSRUFTR1RCUCJVyT+hVmdYw2UiJXYldiJfaTEnTsNlIiVWJVYiX2kxJ1XDOlVyIk58VSIgo1ZhWA==&input=IkVOSU5BTklTVVBVUiIK)