SSH ("Secure SHell") is a text based way to connect to a remote system, that does not involve a web browser. Instead you use a SSH client application.
If your PC runs WIndows, you'll need to install a SSH client such as PuTTY or Tera Term.
Desktop Linux already includes a CLI SSH client called 'ssh'.
Connect to the router using ssh to its address 192.168.1.1.
If you've forgotten the password or other configuration you can also try to get the router into failsafe mode. Plug in the power and watch it boot up (OpenWrt). When the "gear" LED starts blinking rapidly, press the reset / WPS button several times repeatedly and see if it starts flashing very rapidly.
The "factory" file is only used to flash over factory firmware. If you are running any version of LEDE or OpenWrt you would use the "sysupgrade".
But when the version you are changing to is significantly different than what is installed, you should choose NOT to try to save the settings. Instead make a clean install with default settings and reconfigure the new version from scratch.
If your router is issuing DHCP addresses, OpenWrt is doing something, it is not bricked.
I even save a backup of LEDE before installing sysupgrade because of the setting but as discribed the settings usually should be adopted. But I wanted to do it safe. Also the settings I would not care.
The main thing is a stable os latest version that I can control via LAN finally works.
You'll need to install and configure a TLS-enabled version of the web server (typically uhttpd) to be able to connect over HTTP-S. If nginx is available, I would go with that as it is more "production quality" than is uhttpd. It is relatively new and I don't know if it is available for 18.06.1 release devices yet. At least on master there is now a package that loads all that you should need -- https://openwrt.org/packages/pkgdata/luci-ssl-nginx
Of the firmware than can run on an Archer C7, I consider OpenWrt to be be best choice due to its flexibility, package availability, sane build environment, and, most importantly, that it is regularly patched for security issues.