I made an alarm system with this sensor for my bike but i have to turn it on manually. I activate the alarm system always from my tablet or phone when i get home but i would like to automate the activation. How can i make a availability sensor or do you have any ideas how to do it? The sensor does not show when it is connected again.
Is it impossible or is this a stupid question? 
I don’t think the question was stupid, just a bit unclear. Does the sensor show up as “unavailable” in HA, and you want something to happen when it becomes available?
Screenshots and code snippets are a big help as well, so we can see what you’ve tried and the information you’re dealing with. Check out this pinned post, and try following those guidelines when asking a question. I’m sure many people would be happy to help at that point! 
No the sensor does not show up as unavailable when it is out of signal range. In fact it does never show as unavailable.
My idea was to make an alarm system for my bike that would turn on automatically. Currently i have to turn on the alarm manually every time i arrive home. So my question is that how can i see when the sensor is connected again if it is not showing available or unavailable?
My sensor is Xiaomi Aqara vibration sensor DJT11LM
Here’s a relevant portion of that pinned post I mentioned. I really recommend you take a look, or nobody is going to help you.