Try this:
In your wp-config.php file, place the following line, near the bottom of the file, just before the line “/* That’s all, stop editing! Happy blogging. */”
define('WC_CC_ADVANCED', true);
Then, in the WooCommerce settings for CardConnect, at the bottom of the page, you’ll see new settings fields for advanced users and troubleshooting.
You can either turn off the “Enable IFRAME API” checkbox altogether and revert to a non-iframed version.
Alternatively, you can turn off the “Autostyle” checkbox, and and see if you can style it yourself with custom CSS, although styling a Iframe my get blocked by CORS – https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS.
These extra settings are all checked by default, and the default for the “Timeout” field is 500 – just in case you want to revert to the defaults.
If you cannot get it working using the above, respond here, and I’ll see what other options are available.