Wait until the time limit you provided is up. Be patient and do not pursue any further action until the time limit you provided in your initial letter is up. If this date passes and you still haven't heard anything, you may follow up with a phone call or email to check if the letter was received. It is always best to give the company the benefit of the doubt.
If you still do not receive any information regarding your letter or if you do but the situation was not handled to your satisfaction, you can proceed by addressing your complaint to someone higher on the chain of command.