Just a follow-up. Copied an incorrect envelope in the original post. Here's the correct envelope with the namespaces.
<s:Envelope xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Header xmlns="https://bingads.microsoft.com/Reporting/v13">
<Action mustUnderstand="1">SubmitGenerateReport</Action>
<AuthenticationToken i:nil="false">TOKEN</AuthenticationToken>
<CustomerAccountId i:nil="false">CUSTOMER_ACCOUNT_ID</CustomerAccountId>
<CustomerId i:nil="false">CUSTOMER_ID</CustomerId>
<DeveloperToken i:nil="false">SANDBOX_DEV_TOKEN</DeveloperToken>
</s:Header>
<s:Body>
<SubmitGenerateReportRequest xmlns="https://bingads.microsoft.com/Reporting/v13">
<ReportRequest i:nil="false" i:type="AudiencePerformanceReportRequest">
<Aggregation>Daily</Aggregation><Columns i:nil="false"><AccountPerformanceReportColumn>AudienceName</AccountPerformanceReportColumn><AccountPerformanceReportColumn>Spend</AccountPerformanceReportColumn></Columns><Filter i:nil="true"><AccountStatus i:nil="true"></AccountStatus><AdDistribution i:nil="true"></AdDistribution><DeviceOS i:nil="true"></DeviceOS><DeviceType i:nil="true"></DeviceType></Filter><Time i:nil="false"><CustomDateRangeEnd i:nil="false"><Day>14</Day><Month>11</Month><Year>2021</Year></CustomDateRangeEnd><CustomDateRangeStart i:nil="false"><Day>12</Day><Month>11</Month><Year>2021</Year></CustomDateRangeStart><PredefinedTime i:nil="true"></PredefinedTime><ReportTimeZone i:nil="false">CentralTimeUSCanada</ReportTimeZone></Time></ReportRequest></SubmitGenerateReportRequest>
</s:Body>
</s:Envelope>