Anyone with one of these printer/scanners which get infrequent use will probably have experienced the situation where it drops into "power save" mode, which appears to be an HP synonym for "off".
No amount of jiggery pokery can wake it - you need a power cycle.
Best way to fix the issue, is to get a cheap mechanical segment timer, and set it to be on 23 hours 45 minutes of the day, with a 15 minute outage at a time of your choosing (mine is 3am).
Hopefully the Gods of Google will help people by picking up the thread title for people looking for a fix, rather than a conversation with other people with the same - unsolved - problem.