Tolino Tab 8 doesn't boot - recovery mode not working - how to flash

Hello!
I am having problems with a android device. Namely Tolino Tab 8

It does not boot.
On start the error message reads
"Secure Boot Violation - Invalid signature detected. Check secure Boot Policy in Setup"

Then I can't do anything but turn it off again.

When I try to do a hard reset / enter the recovery mode via VolumeUP+PowerOn there is no difference. It behaves like I just turned it on with the PowerOn Button.

So I want to flash it via Dnx Mode (VolUp+VolDown+PowerOn). That's the only action I can perform on this thing.
I downloaded some android 5.0 recovery package from medion.com but on windows and on ubuntu I get error messages because of fastboot (on windows it says "fastboot.exe wrong file or filename" on ubuntu "bash: ./fastboot: No permission")

Can anyone help me with proper instructions on how to flash the thing? Somehow I am too dumb to find it by myself. Sorry.
Thank you very much for your time and every effort

 

Thank you very much for your respond and your time spent on my problem!

I bought the thing in that state. I have no idea what happened. When I went home I was suprised it couldn't boot.

On the medion.com thing: I think those are custom recovery images of android versions, yes. I just picked a thing I imagined would fit (KitKat. 5.0). I was not aware it had to be so specific for the device.

So it's not possible to flash this thing? Would the manufactorer give me the Firmware by any chance?
shoot... I just bought the thing. That's unsatisfying

 

Thank you for your responses!
I want to contact the brand for this, but I still don't know what I actually need.

My questions are not quite answered
Could you tell me in what form I need the firmware? (what datatype? like the wrong android version I had with flash.sh/flash.bat?)
And then tell me how to flash it? There must be tutorials for this...
As I said in my initial post there is a basic problem with fastboot and I don't think that's because of the wrong android version.

FYI: I bought it second hand labeled as defective. Battery was dead and the seller couldn't tell what the problem was. I thought i just would do a hard reset and the damn thing would be fine, but as it seems I gambled and lost.

@Milo Williamson I corrected the title Tolono is very small.. what a coincidence