They both work on that port. I also checked the connection with Postman but get no response from the battery.

You can try to run

sudo nmap -sUV IPaddress -p 30000

or, to scan a range of ports:

sudo nmap -sUV IPaddress -p 28000-30000

(filling in your IPaddress of course)

and see whether it report the port open/filtered or closed.

On my system it comes back with the port open and reports the MAC ...

Yes, I enabled the open API in the Marstek app. and rebooted the battery just in case.

Now this message shows that it probably gets no data from the Venus:

2026-02-15 11:14:03.200 Marstek Venus C: get mode data received: None
2026-02-15 11:14:03.200 Error: Marstek Venus C: No data received during ...

I tried it with a Venus C 2,5kWh and get this error:

2026-02-14 11:13:11.170 Marstek Venus C: es status data receivedNone
2026-02-14 11:13:14.060 Error: Marstek Venus C hardware (7) thread seems to have ended unexpectedly