Hi @slh and @frollic,
I successfully installed OpenWRT using the TFTP method.
I figured out that the commit message is wrong. Installation is currently not possible via the OEM web interface. The switch needs to be opened and you need to attach to the unpopulated serial interface in order to enable TFTP boot.
See: Support for RTL838x based managed switches - #1279 by exponentialverteilt
Thank you for your help!