It’s been a while, since my last install – but I’m fairly sure, the installer should work fine without any network connection. All the RPMs should be on the USB.
Searching the forum for “Ventoy” I find your post from December 2023:
So maybe it is Ventoy and Qubes OS that still doesn’t play well together? – Can you - as a test - try and write the ISO directly to an USB stick?
