Hi Jerome,
Just purchased a RUT950 and attempting the same action, install zerotier and lost connection to the UI. Backend data traffic works via mobile network but the login UI don't continue after successful authentication.
Followed your recommended steps with reapplying the firmware through the bootloader.
This is the checksum of the used file
md5sum RUT9_R_00.07.00.2_WEBUI.bin
335da667707ca0da72bbf42c1e6d41f0 *RUT9_R_00.07.00.2_WEBUI.bin
After pressing Update firmware I get a error about incorrect checksum.
Tried to reinstall the bootloader via http://192.168.1.1/uboot.html and that was successful.
When do you attempt to release a new firmware for RUT950? The last two was withdrawn and this current version is far from good.
Only way to get out of the current situation is to rest the device to factory default.
BTW: Tried to install the DDNS package from package manager and it gives back a failed to fetch package. Maybe it's all related the current issues this firmware suffers from, at least I hope so.
Wish you all a Happy New Year