[SOLVED] New build PC powers up but no display - bad mobo??

[SeattleSUP]

Having a vexing issue with my new PC build. When I power it on the fans start running and the keyboard and mouse light up but nothing outputs to my monitor. My build is:
Asus Prime B-560 Plus motherboard
Intel i5-10400 CPU
2 x4GB G.Skill DDR4-3200 RAM
XFX RX580 GPU (pulled from old build)
Corsair 4000D case
EVGA 500w modular power supply

It's not the monitor - I'm using the same monitor with an HDMI switcher.

Troubleshooting steps I've tried so far:
Checked the CPU fan, panel connectors, ATX12v and ATX power cables are all properly seated
Swapped HDMI cables
verified RAM is correctly seated
removed RAM and used just one stick (tried both to rule out a bad stick)
removed GPU and replaced it with another GPU
removed CMOS battery, let sit for 10 min.
Uninstalled the mobo, set it on a cardboard box and reattached all power and header cables

Any other ideas??? I've built up several PCs over the years but this is the first time I've had this issue.

 

[Colif]

checked display port? could be outputting bios to DP instead.