The purpose of this study is to find the highest dose of the investigational immunotherapy drug TJ033721 that can be given safely in people with advanced cancers of the pancreas, esophagus, stomach, or junction between the esophagus and stomach. TJ033721 is an antibody that binds to Claudin 18.2, a protein expressed on some cancers cells, and to 4-1BB, a protein found on immune cells. TJ033721 may strengthen the immune system's ability to fight cancer cells by activating a patient's own cells to destroy the tumor. It is given intravenously (by vein).