okay, so I downgraded to the chinese 1.0.17 again, reset to factory defaults, gained SSH-access again and upgraded to INT 3.0.22 using the xqrepack-method.
Then I changed:
Luci to appear in english:
uci set luci.main.lang='en'
uci commit
Fixes to keep SSH, enable bootwait and serial access:
nvram set CountryCode=DE
nvram set ssh_en=1
nvram set uart_en=1
nvram set boot_wait=on
nvram commit
I patched the file /etc/config/miwifi to replace all miwifi.com-hostnames by localhost:
# as a last-ditch effort, change the *.miwifi.com hostnames to localhost
sed -i 's@\w\+.miwifi.com@localhost@g' /etc/config/miwifi
Then I added the country-code to the /etc/config/wireless-file:
uci set wireless.wifi0.country='DE'
uci set wireless.wifi1.country='DE'
uci set wireless.wifi2.country='DE'
uci commit
Finnaly I created a script that manually adjusts the regulatory domain by issuing the command "iw reg set DE"
Create the file /etc/init.d/iw-reg-fix and insert this content:
#!/bin/sh /etc/rc.common
# Example script
# Copyright (C) 2007 OpenWrt.org
START=10
STOP=15
start() {
echo start
# commands to launch application
iw reg set DE
}
stop() {
echo stop
# commands to kill application
iw reg set DE
}
Then make it executable:
chmod +x /etc/init.d/iw-reg-fix
And enable it to be exectued on startup: /etc/init.d/iw-reg-fix enable
In the end, I have all the wireless-settings on what I need for operation in Germany (DE):
root@XiaoQiang:/etc/config# iw reg get
global
country DE: DFS-ETSI
(2400 - 2483 @ 40), (N/A, 20), (N/A)
(5150 - 5250 @ 80), (N/A, 20), (N/A), NO-OUTDOOR, AUTO-BW
(5250 - 5350 @ 80), (N/A, 20), (0 ms), NO-OUTDOOR, DFS, AUTO-BW
(5470 - 5725 @ 160), (N/A, 27), (0 ms), DFS
(5725 - 5875 @ 80), (N/A, 14), (N/A)
(57000 - 66000 @ 2160), (N/A, 40), (N/A)
The router switches the 5GHz-channel to 100 and 160 MHz 
The stock firmware even support some cool wireless features for better roaming:
wireless.@wifi-iface[0].bss_transition='1'
wireless.@wifi-iface[0].ieee80211k='1'
wireless.@wifi-iface[0].ieee80211v='1'
