Someone actually suggested unplugging the ethernet cable and reversing it?
It's keyed to work only one way!
I've seen some crazy advice before, but....SMH.
Those plugs are very temperamental, as well as delicate, and can cause all sorts of issues when not well seated. They can also easily work themselves loose if the modem is moved around a lot. Glad the fix turned out to be pretty simple.
Can't remember what renditon of modem I first learned not to mess with the DIN power plug. I originally thought it was convenient to reboot by unplugging that until one time it generated a small but significant spark when I plugged it back in. It worked after that but never went that way ever again. Always used either the wall socket or the one end of the brick after that.