Thanks for answering. Actually the Huawei side is the client side, so no need for incoming connections. In fact, the LTE ISP doesn’t allow for incoming connections.
The server side is a Zeroshell put in DMZ. There are other servers (Web, Mail etc.) running behind it with proper port translation and firewall rules, and all is working.
I tried with regular OpenVPN on Debian, setup as server behind the Zeroshell router, and another one setup as client on the Huawei side, also behing the Zeroshell router, and it is working.
Seems there is a setup issue in Zeroshell when configuring the TCP Server side.
btw, running Zeroshell 3.4.0 on the server side, because newer versions don’t boot. (Windows Server 2012 R2 Hyper-V).