1. Use a Proxy or Backend Service

Solution: Set up a server-side proxy that handles the iframe content. The proxy can handle cookies and make requests to the desired URL. This way, the cookies are managed by the backend, and your iframe can receive the necessary content without directly involving client-side cookies.
How to implement:
Set up a simple proxy server (e.g., using Node.js with Express).
Point the iframe src to your proxy server, which then fetches and serves the content with cookies.

2. Custom Headers via a Backend

Solution: If you control the backend of the iframe content, you can manage cookie transmission there. By using custom headers and sessions, you can pass necessary data without relying on cookies.
How to implement:
Modify the backend to accept custom headers instead of cookies.
Add these headers to the requests originating from the iframe.

3. Switch to HTTPS

Solution: The most robust solution is to switch your site to HTTPS. This enables you to take advantage of SameSite=None; Secure for cookies, which is required for cross-site requests.
How to implement:
Obtain an SSL certificate (many free options are available, like Let’s Encrypt).
Configure your server to support HTTPS.
Update any URLs and resources to use HTTPS instead of HTTP.

4. Use JavaScript Workarounds

Solution: Although less secure, you can use JavaScript to transfer cookies by embedding the content directly instead of using iframes.
How to implement:
Use document.cookie to manually set cookies before the iframe is loaded.
Consider loading the iframe content using XMLHttpRequest or fetch, then injecting it into your page.

5. CORS and JSONP (if applicable)

Solution: If the iframe content is hosted on a server you control, you might be able to implement CORS policies or use JSONP for cross-origin resource sharing.
How to implement:
Modify the server to allow CORS with the appropriate headers.
Use JSONP as a workaround for simple GET requests.

6. Refactor the Architecture