Make sure the web server is bound to all IPs uci show uhttpd | grep listen_http
Files to be preserved depend on the following:
/etc/sysupgrade.conf - customizable backup configuration.
/lib/upgrade/keep.d/* - system configurations provided by specific packages preserved by default.
opkg list-changed-conffiles - list of files derived by package manager.
You can run firstboot from the SSH or Restore Factory Defaults in Luci. No need to flash again.