Third party system to SAP PO single stack Java via RFC

Hi together,

I have a requirement to send IDocs from a non-SAP third party system (which uses a RFC library) to SAP PO single stack java via RFC.

Here's what I've done to achieve this:

On SAP PO side:

- configure a cloned inboundRA in NWA to register a Program ID / TP Name in PO Gateway:

- configure a IDoc Sender channel to use this cloned inboundRA

On third party system:

- I set the following parameters:

ASHOST: <PO hostname>

SYSNR: <PO sysnr>

TPNAME: <Program ID>

USER: <user>

PASSWD: <password>

On my third party system I get the following error:

Fehler beim RfcOpenEx: key=RFC_ERROR_COMMUNICATION message=CMALLC : rc=20 > Connect from SAP gateway to RFC server failed
Connect_PM GWHOST=<PO hostname>, GWSERV=<PO gw service>, SYSNR=<PO sysno>

LOCATION SAP-Gateway on host <PO hostname FQDN> / <PO gw port>
ERROR Gateway not connected to local R/3
TIME Wed Jul 16 16:28:53 2020
RELEASE 753
COMPONENT SAP-Gateway
VERSION 2
RC 726
MODULE /bas/753_REL/src/krn/si/gw/gwxxrd.c
LINE 8564
COUNTER 86752

What am I missing here? What could be the problem? How to get this connection working?

Thanks in advance!