I'm using HDMI-HDMI cable, no mixed connectors nor adapters. But I already solved the problem. I had to disable Secure Boot, and check Enable Legacy ROM option in UEFI setup. This GPU seems to stop boot sequence with Secure Boot. Everything works fine now. Maybe this can help someone.
Hi, my optiplex 3050 won't boot after GPU upgrade. My specs: CPU: i5-7500 Ram: 16gb ddr4 GPU old: NVIDIA GeForce 1030 GPU new: AMD Radeon RX6400 Psu: 260w Everything works like charm on my old GeForce GPU, but when I install new Radeon GPU and turn pc on I just get blank screen (no signal on the monitor), connection via HDMI. I tried uninstalling NVIDIA drivers but it didn't helped. Weird thing is when I connect HDMI to onboard graphics (with Radeon GPU installed) I can see Dell boot logo but nothing more, when I try to enter BIOS or one time boot menu It's just saying "preparing to enter" and freezes. But there is just black screen when I connect to Radeon gpu. GPU seems to work, I can see fan spinning and card is getting warm. Can you help me to figure out what's the problem? Sorry for my english but I hope you can understand me 
Optiplex won't boot after gpu upgrade