Dear Sirs,
putting lte interface down, is there any traffic? My pricing plan says, if there is *any* bit or byte flying I have to pay the full month fee. So, the question is raally money relatet...感谢在adavance.

Nope.
Interface down is interface down.

You can always test in a month where you are using that device

Fair question. If your carriers is looking only looking data sessions, then you should be okay. But if the modem is powered on, it's hard to say that the modem alone isn't doing something that might trigger outside of RouterOS - like some FOTA or other network operation that might "count".

Hi,
thank you for your answer.
I try to ask the ISP what the mean with "any traffic". Not much hope the really have an answer.
Any chance to disable the modem?

I think you'll just have try for 30 days... Really hard to know.

But my thought is if the modem is powered up, the tower sees it... And some newer modem may do FOTA or other queries.

I'd hope they use connection to the customer APN via a data session to track – which disabling the LTE interface should prevent.

You need to configure it so that traffic is regularly sent out both interfaces. The easiest way to do this is to permanently configure one tunnel to use one interface, and the other to use the cellular interface.