Tablet not reachable

Hi,
we use the Sophos Mobile 9.7.3

I have 100 managed Android tablets, Samsung A7. Some of them make problems with the sync.

The tablet in my example was last used in January, and it has an unlock-password (PIN), due to company restrictions. The user how set it, left the company, so we had no password and nobody used it. 2-3 months ago I was contacted and i resetted the password with the Sophos Mobile console.

Now i have this tablet on my desk. It is connected with our company WLAN, as it should, but it will not receive any information. The time on the device was 4-5 hours behind the correct time. This happens really often and if it is more than 5 minutes, no app is working, but this could be an internal network thing.

Back to the password and sync: The reset is not working, because this command is not delivered to the device. I tried so many things, but nothing is working.
I also used an USB-C adapter to connect it via LAN (free network), but it is still not reachable from the MDM. I waited some hours and as I watched again, the time sync work, however, but here is the fun fact.
Tablet shows "connected via LAN, only charging". The WLAN symbol is not there at this time. But when it has no internet, how the time sync with internet or our internal timeserver was possible? But when this was working, why I can not send commands to it via the Sophos Mobile?

I also can not restart, shutdown or change the settings, because of the password protection. Is there any way to "hard reset" the tablet? Because the system functionality is also blocked from Sophos?
Do I need now several weeks till the battery is empty? 
What possibilities do I have to re-manage this device?
Is there any experience with it? My search didn't work for me, so far.

If this is something you can not share with everybody, you can also PN or mail me. I also can prove everything you want :)