Power Mac Pro does not load Installer

Hello,

I recently installed macOS elCapitan on an old Mac Pro G5 I purchased on EBay. Unfortunately I ran a software update which resulted into a boot-loop after installation and restart. Form there on I tried the recovery mode a thousand times to re-install the elCapitan OS again. Somehow the old macOS 10.6 recovery (it's snow leopard I think) was additionally installed on the disk. Due to that I completely formatted the entire hard drive with an external SATA adaptor via my MacBook and plugged it back into the Mac Pro. The hard drive is now completely empty. After preparing a macOS elCapitan installation USB-Drive, I tried to re-install the OS via booting from USB stick.

The issue now is, that I can select the USB Stick from the boot menu, but as soon as I hit ENTER, only a gram screen appears and nothing happens. Maybe somebody of you has an idea what I can try to install the OS again? I switched harddrives, tried different drive slots within the Mac Pro, different USB Sticks for startup - so to speak I have absolutely no more ideas to figure out what's the issue with that Mac.

Thank you for any Ideas!