For anyone reading this after a failled upgrade, here is how you put the R7500 in "put" mode:
With the router turned off, use a sim tool or paper clip and press the reset button.
Maintain pressed while turning on the router.
Power led will flash red, then flash white.
Now release reset button and press PUT on TFTP utility.
Sad that it's a 2018 firmware but holy cow the coverage is amazing...