Xiaomi Wifi Router 3G - 18.06.x / Wifi issues 2.4GHz + 5GHz

Does anyone have the same Problem as me?
After flasing OpenWRT and connecting my routers lan to the wan port of the mi wifi3g I can activate Wifi and its working. But when I turn off the power and turn it back on I cant get internet access over Wifi anymore and it never comes back. I am trying since 10 hours and I really do not know what to do anymore.