Find what values are not in multiple columns
Try this:
with tRMS_CODE as 
   (SELECT DISTINCT RMS_CODE FROM RMS_Table)
   , tRMS_ALT1_Cde as 
   (SELECT DISTINCT RMS_ALT1_Cde FROM RMS_Table)
SELECT DISTINCT * FROM RMS_Table
WHERE 
   SUBSTR(CHILD,6,4) NOT IN (SELECT * FROM tRMS_CODE) AND 
   SUBSTR(CHILD,6,4) NOT IN (SELECT * FROM tRMS_ALT1_Cde) AND 
   APC_Spec='36559'