Hello Antonio,
Welcome to Microsoft Community.
If the Ugee M708 v1 after-sales support clearly tells you that the device supports Windows11, I believe that their product (whether it is a Tablet or a Pen) should work on Windows11, so the possibility that the device is not compatible with Windows11 has been ruled out (unless, After sales cheated you).
In your subsequent description, the pen only works for a few minutes. If you assume that the device is connected to multiple external devices, you can actually do a test for this. Unplug all the peripherals except for the Tablet, and verify that it is working properly. By doing so, you can test whether there are too many peripherals connected to affect the normal use of the Tablet.
However, I think this possibility is very low, and a large probability of this problem will still occur.
If it's convenient, you might want to test connecting the Tablet to another Windows 11 device, install the appropriate driver, and see if it works on another Windows 11 computer.
If you find the problem still occurs. I think you need to bring this back to Tablet support and see if they can suggest anything other than a driver, because it's not a Microsoft device and I don't know much about it. If necessary, the device may need to be sent to the aftermarket for inspection (hardware failure is highly unlikely, as it will work in Windows 8).
If anything is unclear, please do not hesitate to let me know.
Best regards,
Mitchell - | Microsoft community support expert from MSFT