How to set monthly subscription licence, and other Q

And another Q: How to disable offline activation?

The way you disable offline activation is by not accepting activation requests. That part of the process is entirely in your control.

>> "other question is, i deactivate my licence on server and don't deactivate in my software, how is possible, that it still works?"

This is described on the page right below where you click "Deactivate". Short answer: the customer may or may not be connected to the internet thus clicking "Deactivate" remotely is the first step to deactivating that customer from that computer.

>> "Is it due to GracePeriodLength??? Can i remove GracePeriodLength? "

Nope, you shouldn't.

>> "Hi, I'm new here and I wanna ask you how to set up monthly subscription licence in my software and here on server? And if is it possible what are recommended settings?"

Use custom license fields: https://wyday.com/limelm/help/license-features/