Signing retired fighters as trainers

How can you sign boxers you recently retired as trainers in your gym

Usually when they retire it will say they want to be a trainer, either creating a new gym or sacking your current trainer, if your gym is in the same location as their nationality, the retired boxer will appear in the trainers list.

I signed my former undisputed champion Alex Arguello as trainer but mistakenly gave him a gym in Cuba and now he can’t sign any worthwhile fighters. How can I correct that?