How to turn query to querybuilder

I'm working on CI3 project and I'm trying to create a search in datatables all works well when I'm using this code:

 function getDataUserByFilter($rowno, $rowperpage, $keyword = null, $operator = '', $orderBy = '', $sortingBy = '', $filter_data_by_page = '')
{
     $selectQuery = 'select  a.id, a.phone, a.email, a.create_date , a.name, a.status, a.online, a.kuota_task,a.wallet from users a';

     if ($operator != '') {
         $selectQuery = $selectQuery . ' inner join telco_prefix on 
         substring(a.phone, 1, 4) =
         telco_prefix.prefix and telco_prefix.telco_id  = \'' . $operator . '\'';
     }

     if ($keyword != null) {
         $selectQuery = $selectQuery .
             ' where a.phone like \'%' . $keyword . '%\'
 or a.email like \'%' . $keyword . '%\' 
 or a.name like \'%' . $keyword . '%\' 
 or CAST(a.kuota_task AS text) like \'%' . $keyword . '% \'
 or CAST(a.wallet AS text) like \'%' . $keyword . '% \'
 or CAST(a.create_date AS text) like \'%' . $keyword . '% \'';
     }

     if ($filter_data_by_page == 'online') {
         if ($keyword != null) {
             $selectQuery = $selectQuery . ' a.online = 1';
         } else {
             $selectQuery = $selectQuery . ' where a.online = 1';
         }
     }

     $query =  $this->db->query($selectQuery . ' order by ' . $orderBy . ' ' . $sortingBy . ' limit \'' . $rowperpage . '\' offset \'' . $rowno . '\'')->result_array();

return $query;

but only one problem, that it can only search email, name and phone. Other than that it doesn't work. Like if I try to search the kuota_task or wallet or create_date the search not working. Can anyone please tell me what is wrong with the code or help me to find the solution?