Hey there YorkieLover4Life,
Upon researching your question, we were able to find the next steps to take in this link as it discusses the issue you mentioned: If your external display isn't detected or appears blurry when connected to your Mac
We also recommend contacting BenQ about this to check for things like compatibility with your Mac model, firmware updates for the display, etc.
If after trying the above steps the same issue remains, even if you test a different monitor if possible, then the next step would be to let Apple Support know what's been tried as they'll be in the best position to assist further: Get Support
Cheers!