Cell-Site Simulator Warrant Act of 2021 This bill establishes a federal statutory framework to regulate the use of cell-site simulators. Cell-site simulators (commonly known as Stingrays) are devices that function as or simulate a cell-phone tower to identify, locate, or intercept transmissions from a cell phone for purposes other than providing ordinary commercial mobile services or private mobile services. The framework generally prohibits the knowing use of a cell-site simulator domestically by an individual or entity or the use of a cell-site simulator by an element of the intelligence community outside the United States to conduct surveillance of a U.S. person. It imposes a civil fine on an individual or entity that violates the prohibition and restricts the use of unlawfully acquired information as evidence in a legal proceeding or official proceeding. The framework contains exceptions to permit the use of a cell-site simulator in certain circumstances, such as by a law enforcement agency pursuant to a warrant or by an element of the intelligence community to conduct surveillance under the Foreign Intelligence Surveillance Act of 1978. Finally, an individual who is the subject of unlawful use of a cell-site simulator may bring a private right of action.