Did you take a look at the SAP OData Connector from the app store, link: https://appstore.home.mendix.com/link/app/74525/Mendix/SAP-OData-Connector?
Regarding the reference guide this connector should also be able to use the ECC: https://docs.mendix.com/refguide/sap/sap-odata-connector.
"The SAP OData Connector can be used for all SAP back-end systems that have OData enabled. For ECC, SAP Gateway will be used to expose the traditional BAPI interface as an OData service."
Perhaps this helps.