Resolve GA issue on ios for iframe.

The GA is not working on ios for iframe. I am loading our plugin page from different domain on a client's website using iframe. I have added cookie flag on GTM and pasted GTM code only plugin php page, the events are firing on windows and android whenever I visit the website. I am using UA version of GA. If I go to the plugin page directly it works on ios. But it doesn't work on ios when I go to the website.