Monitor won't boot with Mac mini/eGPU

I've recently gotten a Mac mini (2018), with an external graphics card (Radeon RX 5700 XT). The monitor, a HP VH240a, is plugged into the graphics card enclosure (Razer Core X Aluminum External GPU), which is then plugged into the Mac mini. The Mac is running Catalina version 10.15.7.

What works … most/some of the time … is to turn on the Mac mini, wait for a few seconds, then turn on the external graphics card (the monitor starts on its own when the eGPU is turned on). But with annoying regularity, although the Mac, the eGPU, and the monitor are all clearly powered up, the monitor does not recognize that the Mac is running, and puts itself to sleep. Without the monitor, of course, I can't even sign in or affect anything at all, even to shut the Mac off properly,

Could someone tell me how to fix this problem?