Uploaded the squashfs-factory.bin (changed to .img) firmware via web admin as adviced in the wiki but the router doesn't respond at all now.
No ping on 192.168.1.1 (on a router and direct link with manual ip set). Nothing on nmap or nmrpflash.
I tried netcat for udp... Reset btn doesn't work anymore.
I ended up in the same situation. Flashed the openwrt-18.06.2-ramips-mt7620-ex2700-squashfs-factory.bin via the web interface and now there's no DHCP response and no ping response from 192.168.1.1 if I configure 192.168.1.2 for my PC manually...