Q: iPhone will not rotate to horizontal in ap that only works in that orientation

I downloaded an ap that works only horizontally. After installation, the ap screen will not rotate from vertical to horizontal, and the ap will not let me proceed to next step. I can’t turn off the orientation lock

I tried all the steps on this Apple page. when I have the ap open, no amount of swiping will bring up the Control Center. Tried other aps, and Control Center and  Orientation Lock does come up, and I can turn off and on. Of course for those aps, orientation isn't critical. Doesn’t work, however, on the only ap where orientation is essential.

Tried Settings and went to ap. No orientation control.

Turned on and off. Nada.

This is critical. The ap runs an EKG on the iPhone and ONLY WORKS IN LANDSCAPE ORIENTATION.

Seems that Apple knows this problem b/c it has a page on it. Why is there a screen lock, and when does OS decide to lock? Is there anything in Setting to prevent this from happening?

Thanks