Inserting data in the 1:N - Laravel 5 ratio side 1

Question

I have the following tables, Condominios and CondominiosTaxas, that are a relationship 1:N , respectively, my relationship in the model Condominio:

public function Taxas()
{
    return $this->hasMany('WebCondom\Models\Condominios\CondominioTaxa',
                          'CondominioCOD', 
                          'CondominioID');
}

My relationship in the model Condominiotaxa:

public function Condominio()
{
    $this->belongsTo('WebCondom\Models\Condominios\Condominio',
                     'CondominioCOD', 
                     'CondominioID');
}

What happens is this, I need to register the condo fee on a form, and when registering the condo, choose the N rates I want.

However, when making the association, I do not know what to do, because the method sync() exists only in a relationship N:N, I tried to create the condominium, and in it call my relationship and the method sync() passing the rate array, but the error is generated:

Call to Undefined method Illuminate Database Query Builder::Sync()

I wanted to know how can I do this without having to create an N:N relationship, remembering that in this case, although they are fees, a condominium fee has to be independent of another condominium, because if a condominium changes its (own) rate, it cannot affect other condominiums.

Field rates on the form:

<div class="col-md-6">
   <div class="form-group">
     <label for="Taxas" class="control-label">Taxa</label>
     <select name="Taxas[]" id="Taxas" class="form-control" multiple="multiple">
       <option value="" selected disabled>---Nome taxa - valor taxa---</option>
       @foreach($taxas as $taxa)
          <option value="{{ $taxa->CondominioTaxaID }}">{{ $taxa->Taxa }}</option>
       @endforeach
     </select>
   </div>
</div>

Fees arrive as follows in the controller:

array:3 [▼
    0 => "1"
    1 => "2"
    2 => "3"
]