A sysupgrade of 19.07.2 went bad and I had to use TFTP recovery, which was a learning exercise.
The documentation says the router can install Openwrt via TFTP but actually this fails.
I tried using 18.07.8 (last known working build) over TFTP, both using *-sysupgrade.bin and *-factory.bin, but whilst the router does download the files, it doesn't successfully install them, leading to soft brick. You have to install the Linksys firmware then install Openwrt via Linksys web interface GUI.
The only binary that did work over TFTP was the Linksys v1.0.0 firmware.
Other useful info: the firmware is downloaded via TFTP by the E1700 within 5 seconds after being powered on.
Note that the router is fully compatible with 19.07.2 otherwise
PLEASE update the device page with the above info. The rest of the things on the page are correct (filename of firmware binary, IP address, etc.)