i am not seeing the issue – check your site in an incognito/private browser. If its ok then the browser cache needs clearing. If its not then theres a chance the CSS has been cached by your router / ISP network.
You can try disabling the WP Rocket cache but it its more likely related to the old iOS not supporting Flex correctly.
A lot of improvements were made from iOS 10 onwards and iOS 9 is no longer supported by Apple.