No Network connection over wifi on Xiaomi AX3600

I wrote my question >>already here<< at the developer thread for the Xiaomi AX3600.
Maybe my question is a kind of off-topic there, so my question was probably lost in the background noise.

I am not able to set up the device as a dumb access point.
The clients simply do not get an IP address. Interestingly, they do get a connection after about 5 minutes of waiting. By then, however, most clients usually interrupt the connection themselves.

The configuration I used for this works wonderfully on my Redmi AC2100 and Mi 3G - but unfortunately not here.

I have used self-compiled builds (from robimarkos repository), the pre-compiled releases from robimarko and also the bloated build from here and also various others linked in the thread.

I have the same effect with all of them.

In order to understand my problem, I approached it with a minimal configuration:

  • clean flash robimarko's prebuilt release image
  • set the following minimal configuration:
config device for dump ap
# don't do router jobs on dump ap:
service firewall stop && service firewall disable
service dnsmasq stop && service dnsmasq disable
service odhcpd stop && service odhcpd disable

# join my lan-network with gateway
uci set network.lan.ipaddr=''
uci set network.lan.gateway=''

# disable dhcp6, ra for ipv6
uci del dhcp.lan.ra
uci del dhcp.lan.ra_slaac
uci del dhcp.lan.dhcpv6
uci set dhcp.lan.ignore='1'
uci del dhcp.lan.ra_flags
uci add_list dhcp.lan.ra_flags='none'

# only bridging - no routing on interface lan
uci del

# wireless settings
uci set wireless.radio1.htmode='HE40'
uci set'40'
uci set'DE'
uci set wireless.default_radio1.ssid='Test'
uci set wireless.default_radio1.encryption='psk2+ccmp'
uci set wireless.default_radio1.key='testtest'
uci set wireless.radio1.disabled='0'

uci commit
wifi reload

Result: The wifi clients simply do not manage to get into the network. The router itself has perfectly fine internet-connection (opkg update etc. works fine).
Ethernet-connected clients work as they should - only the wifi clients are effected.
I also changed the device for the interface "lan" between a bridge over multiple ports and directly single ports like eth0 and eth1 - but no effect here.
I uploaded the client-side wireshark recording >>here<<.