mount
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/mtdblock3 on /overlay type jffs2 (rw,noatime)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
Perhaps something is corrupted in your image. You could download from web again, confirm the sha256 hash, and then upgrade again over tftp. This will give you a known good starting place.
This is why I think something got corrupted... if you have access to the router you could do:
cd /tmp/
wget http://downloads.openwrt.org/releases/18.06.1/targets/ar71xx/generic/openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
sha256sum openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
## check the sha sum against the one published online here: http://downloads.openwrt.org/releases/18.06.1/targets/ar71xx/generic/
# ONLY IF IT MATCHES:
sysupgrade -n openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
You're making this extremely difficult (you've been told about 4 times in 2 threads):
cd /tmp
http://downloads.openwrt.org/releases/18.06.1/targets/ar71xx/generic/openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
sha256sum openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
#COMPARE THE ONE DISPLAYED HERE WITH THE ONE ON THE WEB BROWSER!!!
sysupgrade -n openwrt-18.06.1-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
First copy another copy of the OpenWrt 18.06.1 sysupgrade file to the router's RAM disk under /tmp. You can do this directly with wget as @dlakelan described-- or download the file to your PC then use scp to push it to the router.
Use sha256sum to compute the "fingerprint" of the file. Manually compare that to the posted sha256 sum in the file on the openwrt download site. If it is the same you can be highly confident that the file was not damaged in transport.
Then run sysupgrade -n to put this file into the router's flash. The router will reboot with default settings.
Allow 5 minutes after the reboot before trying to log in. It needs to format the overlay filesystem in the flash before it is ready to use. Do not cut the power during this time.
If your problem is solved, feel free to mark the relevant post as the solution; and edit the title to add "[SOLVED]" to the beginning (click the pencil behind the topic).