Hey there,
Thanks so much for using Apple Support Communities.
For this particular issue, we would recommend that you try performing a settings reset on your iPhone. This step can resolve many issues including issues with mirroring your iPhone screen to an external monitor. The steps are pretty easy and can be found here: Return iPhone settings to their defaults - Apple Support
One thing that is important to note is that if you choose "Erase all content and settings" by mistake, it will erase the data on your device as well as the settings. By choosing "Reset all settings", it will simply revert your device settings back to default. This includes things like network settings, the keyboard dictionary, location settings, privacy settings, and Apple Pay cards.
We hope this does the trick for you!
Cheers.