Incorrect iframe implementation

Some context, we are using a third party authentication platform where ecommerce sites can choose to display this platform within an iframe during their checkout process. We have media queries that detect the iframe size and thus would tell the site which size/screen design to display. The problem is, sometimes and we do not know for sure why atm, the screen will appear cut off within the iframe. Attaching image for clarity. The user is unable to scroll or drag the screen to get to the CTA and will feel compelled to abandon the transaction.

Now, we feel strongly that this is a technical issue, but I am still being instructed to provide a design (visual) solution(s). For example, adding in directional arrows to the UI so a user can still navigate in the case scrolling is not enabled. Has anyone ever had to deal with something like this? Any advice for designing or implementing iframes?

Thanks!