vPro/iAMT, BIOS does not show up via serial console (SOL)

I've setup our desktop machines with vPro/iAMT 11.0 for SOL (serial console via LAN). It works fine while an OS is running.

However when rebooting the POST screen is not visible via SOL and I'm not able to enter BIOS setup. I've tried several BIOS settings (console redirection to com0) but no luck. Maybe I need to configure iAMT too, to redirect com0 somehow? I don't find something useful in the "Intel AMT Implementation and Reference Guide".

I think it should work, regarding this article https://software.intel.com/en-us/articles/using-intel-amt-serial-over-lan-to-the-fullest:

"A more typical use is to use this serial port along with a BIOS feature that redirect the screen text output to the serial port. This allows administration consoles to remotely view what is going on during boot, access the BIOS screens and change settings or boot to a simple OS [...]"

There is also a nice picture in that article how it should work ;)

Please point me into the right direction.

BTW I'm using Fujitsu D3402-B mainboards.