Solution:
You may use whereHas
method which allow to define additional query constraints on your relation. E.g:
$searchText = $request->searchText;
if ( explode( '.', $request->searchColumn )[0] == 'customer' ) {
$query->whereHas( 'customer', function( $query ) use ( $searchText ) {
$query->where( 'name', 'LIKE', '%' . $searchText . '%' )
});
}