I've got a new AC1750 v5. It gives an invalid file with the web UI. The TFTP server method (using openwrt-18.06.4-ar71xx-generic-archer-c7-v5-squashfs-factory.bin) isn't working for me either. I can see the file upload when pressing the power with reset held down for 15 seconds,but it reboots without openwrt.
I'm using windows 10 with solarwinds tftp. Firewall off. The tftp logs show two successful file transfers. I've set the network adapter on the pc to 192.168.0.66/24 and the file is downloaded by 192.168.0.86, so the sftp part is working.
Archer AC1750 isn't a proper device name.Can you confirm that your device is Archer C7 not A7? Specs might be similar, but you should ensure that you are using the image for your exact device.
The A7 needs an A7 factory image (from snapshots/targets/ath79/generic). If you don't want to run a snapshot build you can then (from OpenWrt) sysupgrade to the 18.06.4 ar71xx release C7v5 firmware, since the hardware is the same.
But the TFTP looks for an A7 header and that is why the C7 build will not flash that way.
