I had this issue, using a Razor Core X. I solved it with a HDMI plug. You can read about it at this link:
I ordered one from Amazon and it arrived today. And it solved my problem. My set up, a 2018 Mac mini i7, 500 GB, 32 GB RAM. I have a Razor Core X eGPU with a Sapphire Pulse RX 580 GPU that I took out of my 2010 MacPro when I transitioned to the Mini. I have two ancient Apple monitors, a...
forums.macrumors.com
There is additional info on how to set up the plug to mirror displays further along in that thread.
I have since acquired an old TV, with HDMI ports, and now I have that connected to the HDMI port on my Mini, giving me three monitors, two on the Razor Core X and one directly connected to the Mini.
Good luck.