I'm not familiar with ERPNext, but to do both POST and GET requests you will need to create separate REST connections and call each of them separately.
Where you may well hit a problem with the REST connector in Qlik Sense Cloud is that generally you need to pass a token form the authentication phase to the retrieve phase. Typically this is done by amending the URL with variables.
Depending on what needs to be passed in, and how it is passed in, this amending of the second call may not be possible. In Enterprise (or Desktop) you can turn on Legacy mode, which gives more freedom around how URLs are built.
Hope you can get it working though.