Switch Audio Output while using Lightning Digital AV Adapter on iPhone 7

Hi there! My first post here.

I am testing out my newly bought portable monitor (Lepow 15.6 in) that has its built-in audio output. It is working nice and good for my both Windows 10 laptop and Macbook Pro (macOS Catalina). I was also able to switch the audio output from my laptop (either of the two) to my portable monitor. Which is a nice thing to have for a portable monitor able to do audio output.

However, when I tried this on my iPhone 7, I can't seem to find the setting in which I can switch the audio from my portable monitor back to my iPhone.

I.e. I am playing music from Apple Music and whenever I connect my portable monitor to mirror my iPhone 7, it always redirects the audio out to my portable monitor. That is to say, it is only possible with the use of a Lightning Digital AV Adapter.

I have checked the Settings, the Control Center, everything. But I can't seem to find where can I switch the audio output.

Is there anyone who has encountered the same scenario? I am sure that this has nothing to do with the portable monitor I just bought since it is working on the Windows OS and macOS so suspect it has something to do with the iOS not having this option/feature.

Please let me know if you have any questions regarding my post and I'll do my best to provide more info. Any ideas/guidance will be very much appreciated.