There is no real technical difference between HTTP-RPC and REST. Both use HTTP protocol, with HTTP actions on URLS, with usually JSON messages.
This means that Mendix REST can be used out-of-the-box for HTTP-RPC. It could also be called a HTTP-RPC client, only reason it's not called this way is that HTTP-RPC is not very popular anymore.
So, what is the difference between REST and HTTP-RPC when it's not technical? It's a best practice: the URL in REST normally refer to data, where as the URL in HTTP-RPC normally refers to an action.
* To get contract 1 in REST => GET http://localhost/api/contracts/1
* To get contract 1 in HTTP-RPC => GET http://localhost/api/getcontract?contract_id=1 or something similar.
Anyway, the REST consume functionality in Mendix doesn't really care if your URL points to data or a remote procedure (RPC).
Check out https://docs.mendix.com/refguide/consumed-rest-services about consuming a rest service, see also the tutorial video with it!
The documentation seems straightforward. Did you follow this learning path? https://gettingstarted.mendixcloud.com/link/path/33
I have misread the part about RPC. There is a github project that might be usefull: https://github.com/gk-brown/HTTP-RPC/blob/master/README.md
But you might run into more problems: https://stackoverflow.com/questions/3641724/why-is-rpc-over-http-a-secutity-problem
My assumption might be wrong but RPC seems to be on the way out and might not even work in a Mendix environment.