It seems that you set up exroot and then, after exroot is broken, it cannot mount the internal overlay system.

Have you tried to connect the external HD again to see if it fixes?

Reset the router and start over is also the correct way. But seems you failed to reset it.

While the router is running normally, press and hold the reset for 10 seconds (LED flashes 4 times per second) then release finger.

In ssh, just type “firstboot”. Follow the guide to clear everything, then reboot.