i go to change the router openwrt 18.06.1 very bad unstable more and more problem original firmware is stable
has reconnect problem with ppoe
try install openwrt 17.01.6 from the web browser http://192.168.1.1
cant install cant down from 18.06.1 to 17.01.6
try install development firmware upload by http://192.168.1.1
the router install more and more time hangup type please wait for 10 minutes.
i make manual restart the router web http://192.168.1.1 not open again
try install 18.06.1 by serial cable usb ttl the router is not install the firmware get ( ......... ) big time 7 minutes and not finish install
after used sf write 0x81000000 0x20000 0x$(filesize) take big time and not install
Conclusion With my experience with this system ( openwrt )
not stable very bad
not easy to install
always make big problem some time when try install new Firmware
( Now I need a programmer Flash IC Their price 1600 Egypt Pound to reprogrammer the router the new router TP-Link VR 600 is 1850 EGP the Linksys x6200 1950 EGP )
i go to get new router and not try install this firmware again.
I believe if you visited the wiki you wouldn't have this problem anyway. The development version comes without a web interface so it was just normal that you were not able to access LuCI but SSH would be available which you never tried to access.
The downgrade to 17.01 or upgrade to 18.06 has a little name change for some or most routers. That is why you cant flash from LuCI and you need SSH to do it with -f option.
At the end whatever commands you used I'm not familiar with them but maybe you have just bricked your router and can easily flash it back through serial access and tftp. Again use wiki for this.
the router is totally damage
i buy others Linksys x6200
and not try any firmware again openwrt ddrt etc
only the official firmware is stable other
A waste of time and getting into many problems and a headache