Does anyone have contact info for loans issued by Marcus via Amazon SellerCentral?

Had a line of credit / loan from Marcus (Goldman Sachs) via Amazon / Seller central. Payment went missing, and no one at Marcus has any clue. They claim they sold the loans to a third party, but every third party I have contacted also has no clue. I have tried every number they have given me, with no luck, including 833-906-2224, 833-870-0540, 855-730-7283. Marcus’ customer service is absolutely INFURIATING to deal with. Have spent hours so far on a wild goose chase via telephone.