AVM FRITZ!WLAN Stick AC860 in Debian Buster (10.0)

The FRITZ!WLAN Stick AC860 is a USB-WLAN Dongle compatible with WLAN AC (up to 866 MBit/s) as well as WLAN N (up to 300 MB/s).

 user@computer:~$ lsusb  
 Bus 002 Device 006: ID 057c:8503 AVM GmbH   

A number of websites state that the stick should work out of the box with Linux kernel 4.19. Debian Buster, released July 6th 2019, comes with a 4.19 kernel. Unfortunately the stick did not work out of the box.

• When attached to a USB-port of the keyboard, the device only connected in CD-ROM mode (an AVM gimmick for Windows set-up). When connected directly to the PC, no issue.
• The firmware was missing: mt7662u.bin and mt7662u_rom_patch.bin need to be installed in /lib/firmware/mediatek/ (and/or /lib/firmware/)

This resolved not yet all issues, as there were still error messages related to firmware loading and no wireless device appearing.

• Thus I compiled a current kernel: I am running now kernel 5.1.16
  I don't know which minimum version is required, as I did not test any kernel versions inbetween. A wireless device appeared in the NetworkManager and Wifi-Networks were visible, yet no connection was feasible.
• Adding the following line in /etc/NetworkManager/NetworkManager.conf solved this last problem:

  [device]
  wifi.scan-rand-mac-address=no