Make a custom function call without using SAP NetWeaver

I know a couple of ways how can you can go ahead and use the RFC you have created from SAPUI5 but without using SAP NetWeaver.

You can try to create a web service or a REST service which uses the RFC that you have created. Host the service in the SAP environment and then you can use the service in SAP UI5 with the help of URL.

Or you can go for a SICF service and have a handler for the service. In the handler of the service, you can have the same logic for fetching the content as you have in RFC.

If you are hesitant to go for a service then go ahead with a connector approach. You can use HANA Cloud connector, which can be used easily by the HCP applications. 

You can opt for any of the two to get your implementation in place.