Same problem here.
Main router RT-AC1900p (380.69)
Brigde Router: Rt-AC68u (384.10007)
When i flashed my RT-AC68 (brigde) with the new 384.10007, immediately all the devices hooked on it lost internet access, even the lan.
I have tried everything. clear nvram, initialize, factory reset, reboot .... Always the same little yellow triangle (No internet access)
AC68(bridge) is now back to 380.69, and work perfectly
*It may because that the main Router must be on the .10007 version as well, i have not tried it.
I do not need at the moment the AIMESH stuff for my network configuration. All I need is a bridge that works.
Then my bridge will stay on 380.69 for a moment.