How to configure H803WF module for DMX output

Anyone here experience configuraing H803WF module for DMX out ?
I compile WLED with DMA enabled, neasure the output with a scope, but initially the output seems only to be affected by settings on the LED preferences page, not the DMX page. I do not have a schematic for the H803, but I asume the output for DMX = 1 ( Same pin as used for usart TX during flash)

After power up I see a pulse on the output ( repetition ~ 25ms, width ~1.2ms)
This pulse disappears when I change proxy universe to a value > 0 ( what should proxy unverse be ?
(I just want MQTT → DMX)

Anyone with H803 experience ?

The H803 is designed (before WLED firmware is installed) to accept ArtNet / DMX input from an ArtNet or a DMX controller.

WLED firmware is designed the very same way. It is designed to accept DMX or ArtNet control from an external source.

What I understand, is that it is also possible with WLED to use MQTT to control DMX lights. This is what I try to accomplish.