Hey William,
There’s no error in the console so the only reason I can think of why the Mobile Navigation works inconsistently in older devices and browsers is the device can’t handle multiple Javascript powered displays like the slider powered by Slider Revolution in your page. This is probably why you’re seeing other sites built with X not having the problem you’re experiencing.
Please first test removing the slider in your page then check if the issue persists. If it persists, try updating your browser’s OS to the latest version allowed for your device. Also, update the browser.
Please note that we also follow what Operating Systems and browsers WordPress supports and it looks like iPad 3 is no longer supported. See https://make.wordpress.org/core/handbook/best-practices/browser-support/
Thanks.