Hi Marexaw,
Thanks for reaching out to the Apple Support Communities. We understand you're having issues with your external monitor not being detected on your MacBook Air, and we'd be happy to get you pointed in the right direction.
To get started, we would recommend reaching out to ASUS Support to confirm if the monitor is compatible with your MacBook Air. For help with reaching out to ASUS Support, check out this link here: ASUS Support
If you're able to confirm the the monitor is compatible, check out this next link that can help with troubleshooting this specific issue: If your external display isn't detected or appears blurry on your Mac - Apple Support
Keep us updated on the results.