to upgrade the R7800\D7800 this is quite simple
1, u must plug an ethernet cable to ur pc from ur router.
2, set ur local ip to a static ip 192.168.1.4 (or the default one that the router get after a reset)
3, turn off the router
4, turn on the router and press the reset button untill u see a power white led blinking
5, use a linux machine or ubuntu on windows 10.
6, install tftp (apt install tftp)
7, tftp 192.168.1.1 (or different ip )
8, bin
9, put firmware.img
wait couple of sec, it should be writing send x bytes, the router then will reboot couple of times and thats it 