How do I select main GPU? How can I have output on my Radeon GPU?

Situation: Desktop PC with and AMD CPU with an iGPU plus a Radeon GPU at PCIe. EndeavourOS with systemd-boot (but have also tried Pop OS which uses GRUB). In the BIOS settings, the PCIe GPU is selected as primary. The bootloaders (both systemd-boot and GRUB) give VGA out from both mainboard connectors (iGPU) and the Radeon connector. But when the kernel takes over, output is only available from iGPU. Both for boot messages and later through SDDM, wayland, KDE Plasma. When a monitor is attached to the PCIe GPU, the monitor turns on but has a blank screen. If I disable iGPU from the BIOS, then no output on either GPU.

# inxi -F
System:
  Host: caladan Kernel: 6.9.3-arch1-1 arch: x86_64 bits: 64
  Desktop: KDE Plasma v: 6.0.5 Distro: EndeavourOS
Machine:
  Type: Desktop System: ASUS product: N/A v: N/A serial: N/A
  Mobo: ASUSTeK model: TUF GAMING B650-PLUS WIFI v: Rev 1.xx
    serial: 240131817600271 UEFI: American Megatrends v: 1811 date: 10/07/2023
CPU:
  Info: 8-core model: AMD Ryzen 7 7800X3D bits: 64 type: MT MCP cache:
    L2: 8 MiB
  Speed (MHz): avg: 1429 min/max: 400/5050 cores: 1: 400 2: 400 3: 3657
    4: 4019 5: 400 6: 400 7: 400 8: 3601 9: 3609 10: 400 11: 400 12: 400 13: 400
    14: 400 15: 3584 16: 400
Graphics:
  Device-1: AMD Navi 32 [Radeon RX 7700 XT / 7800 XT] driver: N/A
  Device-2: AMD Raphael driver: amdgpu v: kernel
  Device-3: Jieli USB PHY 2.0 driver: snd-usb-audio,uvcvideo type: USB
  Display: server: X.Org v: 24.1 with: Xwayland v: 24.1.0 driver: X:
    loaded: amdgpu unloaded: modesetting,radeon dri: radeonsi gpu: amdgpu
    resolution: 3440x1440~60Hz
  API: EGL v: 1.5 drivers: radeonsi,swrast platforms: x11,surfaceless,device
  API: OpenGL v: 4.6 compat-v: 4.5 vendor: amd mesa v: 24.1.1-arch1.1
    renderer: AMD Radeon Graphics (radeonsi raphael_mendocino LLVM 17.0.6 DRM
    3.57 6.9.3-arch1-1)
  API: Vulkan Message: No Vulkan data available.
Audio:
  Device-1: AMD Navi 31 HDMI/DP Audio driver: snd_hda_intel
  Device-2: AMD Rembrandt Radeon High Definition Audio driver: snd_hda_intel
  Device-3: AMD Family 17h/19h HD Audio driver: snd_hda_intel
  Device-4: GN Netcom Jabra EVOLVE 20 SE driver: jabra,snd-usb-audio,usbhid
    type: USB
  Device-5: Jieli USB PHY 2.0 driver: snd-usb-audio,uvcvideo type: USB
  API: ALSA v: k6.9.3-arch1-1 status: kernel-api
Network:
  Device-1: Realtek RTL8125 2.5GbE driver: r8169
  IF: eno1 state: up speed: 1000 Mbps duplex: full mac: 10:7c:61:73:9a:73
  Device-2: Realtek RTL8852BE PCIe 802.11ax Wireless Network
    driver: rtw89_8852be
  IF: wlan0 state: down mac: 12:07:7d:d1:bc:35
Bluetooth:
  Device-1: IMC Networks Bluetooth Radio driver: btusb type: USB
  Report: btmgmt ID: hci0 state: up address: A8:41:F4:2B:00:6D bt-v: 5.3
Drives:
  Local Storage: total: 1.82 TiB used: 11 GiB (0.6%)
  ID-1: /dev/nvme0n1 vendor: Corsair model: MP600 PRO XT size: 931.51 GiB
  ID-2: /dev/nvme1n1 vendor: Samsung model: SSD 970 EVO 1TB size: 931.51 GiB
Partition:
  ID-1: / size: 906.17 GiB used: 10.9 GiB (1.2%) fs: ext4 dev: /dev/nvme0n1p2
Swap:
  ID-1: swap-1 type: partition size: 8.8 GiB used: 0 KiB (0.0%)
    dev: /dev/nvme0n1p3
Sensors:
  System Temperatures: cpu: 40.6 C mobo: N/A gpu: amdgpu temp: 37.0 C
  Fan Speeds (rpm): N/A
Info:
  Memory: total: 32 GiB note: est. available: 30.57 GiB used: 4.26 GiB (13.9%)
  Processes: 305 Uptime: 18m Shell: Bash inxi: 3.3.34

Dmesg has this interesting part:

[   23.739334] [drm:amdgpu_bo_init [amdgpu]] *ERROR* Unable to set WC memtype for the aperture base
[   23.739462] [drm:amdgpu_device_init [amdgpu]] *ERROR* sw_init of IP block  failed -22
[   23.739587] amdgpu 0000:03:00.0: amdgpu: amdgpu_device_ip_init failed
[   23.739588] amdgpu 0000:03:00.0: amdgpu: Fatal error during GPU init
[   23.739590] amdgpu 0000:03:00.0: amdgpu: amdgpu: finishing device.
[   23.739712] amdgpu: probe of 0000:03:00.0 failed with error -22