H510 PRO BTC+

Hi guys
I just got my brand new H510 PRO BTC+ and I started installing the following:

!) Intel 11th Gen Core i9-11900KF Rocket Lake
2) KLEVV 32GB (32GB x 1) 3200MHz DDR4 Desktop Memory
3) Hikvision E2000 1TB M.2 PCI-e Gen 3 x 4 NVMe 3D NAND SSD
4) Corsair RMx White Series RM850x 850 Watt Power Supply
5) Samsung S22F350FHA 22" Full HD 1920 x 1080 Business Monitor
6) Logitech K400 PLUS Wireless Touch Keyboard
7) ASUS Strix ROG OC RTX 3090 GPU

All items are brand new and installed very carefully.

I switched it on and hoped for the best. All I got was 5 short beeps, no display on the monitor, no matter if I plug the HMDI into the MB or the GPU. I rechecked the CPU, no bent pins, RAM, SSD, PSU, GPU, and monitor. If I remove everything except the CPU, I get 3 long beeps, which I understand means no RAM. I also understand that 5 beeps mean either no monitor present or a problem with the CPU. I checked the CPU, SSD, GPU, and RAM on another MB and they check out OK. If I put back the RAM I get 5 beeps. Plugging in anything else makes no difference, still 5 beeps.

So now I'm truly stuck, I don't know what to do next. Please help me. Thanks.