Laravel filter nested collection

I have a question that appears to be simple.

I will quickly explain the practical problem:

1. A radio has many coordinates.
   
2. I have a collection of radios.
   
3. I want to filter the coordinates based on a condition.
   

4. The below code doesn't work for some reason.

   $radios = $radios->map(function ($item, $key) {
   
       $coordinates = $radio->coordinates;
       $coordinates = $coordinates->filter(function($coordinate, $key) {
   
           // return true or false;
       });
   
       $radio['coordinates'] = $coordinates;
   
       return $radio;
   });
   
   

I can filter the coordinates collection but can't "attach" the filtered array to the radio object.
What am I doing wrong?

via Jacob Nørgaard