AJAX is not triggering PHP mailer when submitting from iOS

dataString = 'param=stuff';
jQuery.ajax({
   cache: false,
   type: "POST",
   headers: { "cache-control": "no-cache" },
   url: "https://example.com/assets/mailer.php",
   data: dataString, 
   success: window.location.href = "https://example.com/thankyou"
});

This works fine with various browsers on Windows, MacOS, and Android. On iOS, the PHP mailer script isn't triggered, but the redirect works. Using Safari dev tools I can see there aren't any errors in the console. What am I missing?

success accepts a function, you are passing it a string while simultaneously assigning that string to window.location.href and navigating away.

Use a function instead...

jQuery
  .post("https://example.com/assets/mailer.php", { param: "stuff" })
  .done(() => {
    window.location.href = "https://example.com/thankyou";
  });

Note that I've removed all the redundant options you were setting and used the safer method of encoding the request body.

FYI you definitely don't need jQuery for this

fetch("https://example.com/assets/mailer.php", {
  method: "POST",
  body: new URLSearchParams({ param: "stuff" }),
}).then((res) => {
  if (res.ok) {
    window.location.href = "https://example.com/thankyou";
  }
});