After you file

Not sure how a CP2000 for missing income can get you an increase in your refund ... it usually means you OWE the IRS and need to pay them ...  so read that notice again carefully.   

 

If  it really  did say you are due a 2018  refund  then although   I doubt it  will be paid out at this late date  you could try signing it  and mailing it  in as directed ... can't  hurt to try.