Anyway, after that "check", they told me to absolutely not turn on the device when on board...
...And that is perfectly within their authority. This is not a democracy. You are a passenger, subject to the authority of the flight crew. This is governed by Admiralty law. Don't like it? Argue when you're safe on the ground. However, in the air, they will do whatever is needed to keep you from diddling around with something that would affect the safety of flight.
Most people don't realize that their RF toys radiate even when they're receiving. They're "certain" that it can not be the problem, so they light up all sorts of radios and the like, ignorant of how fragile and sensitive the navigation systems can be.
If you look at FM broadcast radios with 10.7 MHz IF strips and low side LO, you'll find that the LO ends up on the VOR navigation band. There have been ASRS reports of passengers with leaky FM radios turned on. The flight crew saw flags on both VOR receivers (indicating a problem), aborted the instrument approach, and then went in to the cabin to see what happened. They found a couple of idiots trying to get the latest on a sports game, The were told to turn off the radio, which they did. The second try to get in to the airport was attempted and then the same damned thing happened.
This time the crew confiscated the radio, completed the approach in to the airport and then returned the radio to the furious idiots who had no idea what danger he put everyone in.
This story is many years old. However, despite the fact that modern radios are usually better than this, the flight crew are not RF engineers, so they have a standing order to all passengers not to light up their electronic toys while screaming down an ILS approach at 150 knots on a rainy, dark, and gloomy night.
Please do not violate that order. This risk is not yours to take. I say this with experience as an instrument rated private pilot and as a registered professional engineer. When the crew orders you to turn off your RF toys TURN THEM OFF!
I freely acknowledge that 99.99% of all RF gadgets aren't likely to be a problem. However you don't know that your gadget is working as designed, and you don't know what navigation systems the airliner is using. Neither you, nor the crew, will be able to know if your toy is causing difficulty in time to do something about it.
Unless you have spoken to the Captain and arranged a protocol to coordinate with the flight crew, keep all gadgets turned off.