Sadly the FRITZ!Box doesn’t allow me to change the channel width. Only change was the mentioned 160MHz. This setting is not on though.
From the documentation
Allow wireless channels with 160-MHz channel bandwidth (5-GHz frequency band)
Increases the maximum radio channel width from 80 MHz to 160 MHz and allows the latest Wireless AC devices with 2 x 2 antenna technology to double their throughput rates.