How to add custom action button

you can try this. change your setting to:

settings = {
hideSubHeader: true,
actions: {
  custom: [
    {
      name: 'yourAction',
      title: '<i class="ion-document" title="YourAction"></i>'
    },
    {
      name: 'editAction',
      title: '<i class="ion-edit" title="Edit"></i>'
    },
    {
      name: 'deleteAction',
      title: '<i class="far fa-trash-alt" title="delete"></i>'
    }
  ],
  add: false,
  edit: false,
  delete: false
}
...
};

then add this into your component.scss

    :host /deep/ ng2-st-tbody-edit-delete {display: flex !important;
  height: 0 !important;
}

:host /deep/ ng2-st-tbody-custom a.ng2-smart-action.ng2-smart-action-custom-custom {
  display: inline-block;
   50px;
  text-align: center;
  font-size: 1.1em;
}

:host /deep/ ng2-st-tbody-custom a.ng2-smart-action.ng2-smart-action-custom-custom:hover {
  color: #5dcfe3;
}