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Thank you for contacting Teltonika Networks Crowd support community.
To recover your device please reset the device. Check the following link on TRB140 recovery procedure.
If you will have any challenge in implementing the solution, please let me know.
If anybody encounters a similar issue with TRB14X devices failing to boot after update to 7.3.x firmware, a thing to note about the referenced instructions link in the original question is that rootfs compression has changed on TRB14X devices since 7.3 version, thus rootfs.ubi file found there is not valid.
For a correct file, please send me your device's product code in a private message, I will provide you with the correct file, as it is dependent on the device code.