Clearly the first thing to do then is to request the affidavit of service.
This seems like a pretty low threshold for serving documents, considering that child support payments could be some $100,000 over 18 years. This isn't a small claims court case.
It is corruption. The federal child support enforcement program is a pork-barrel program that pays-out in proportion to collections. So, states stopped caring about anything that might even vaguely resemble justice of any kind and changed their laws to maximize pork. Civil rights have been eliminated to that end.
Over 18 years, mine would have been well over 5 times that. A half a frickin million bucks. Not too shabby, eh?