A trick for that is to tell them to now try actually unplugging it from the wall and turning it back on again. This gets them to actually do it instead of lying and/or not understanding what it means to actually turn it off and on again
I worked tech support for an ISP, and i did this more often than i want to think about it.
It didn't help that one of the cable modems we gave out to our clients had a standby-button, which made the CM look like it was off - there was no indicator at all on the device, so i couldn't even blame the client for that (but i did blame my employer for not thinking about that. just like i blamed him for buying another modem series with power sockets which failed pretty quickly. did i mention that repairs were done in-house, and not all too well? it's been 20 years, and i am still a bit salty for all my wasted time)
Unfortunately there were no other parties present to provide a second opinion, only their cat. Which, to be fair, is probably less tech illiterate than the human.