Hi! If one monitor works with the mini-PC, but the second one isn't recognized at all via the other HDMI port, it's not necessarily a Windows issue. I would first check whether both HDMI ports on the device can truly be used independently as video outputs.
Please test both monitors individually on each HDMI port, ideally always using the same, known-to-be-working HDMI cable. In Windows, press Win + P and select "Extend," then go to Settings > System > Display > Multiple displays > Detect and check if the second screen is detected.