PHP takes empty captcha session …

Is it possible in PHP?
I have a website with form + captcha on domain A -> there the captcha worked well.
It generates 1 session with the captcha code.
explanation -> https://postimg.cc/Wdzcw8vB
-> no problem so far

Now the server on which the domain A is hosted disabled sendmail - PHP mail() function so no PHP mail() function and no phpmailer with smtp on google works.
This way I can't send forms anymore.
Since it's impossible (for personal reason!) to move the website from domain A on server A to domain B on server B , I decided to use 2 domains on different servers.
I keep domain A on server A for the website and domain B on server B to send the forms -> works good except the captcha.
explanation -> https://postimg.cc/hhsRHvKb
My problem is when I call the form with captcha in iframe on domain B it generates 2 sessions in the /temp folder of server B: one with the captcha code and one empty! Now It always choose the empty session so the captcha is always wrong.

What I don't understand is ...
I open the domain A on server A with an iframe with contents which is on domain B on server B -> no session has been sent at all. Just opening a page from another server B in an iframe on server A.
Now from that page (normal page) which is on domain B on server B, I call a new page with form in the iframe with the captcha -> session_start(); -> so session is sent from domain B on server B
look again for explanation here -> https://postimg.cc/hhsRHvKb
So I call the session on domain B on server B -> why does it create 2 sessions instead of 1
What PHP code do I have to add so it takes the right session with the captcha code?
I hope someone can help me with my problem (if it's possible of course)
Thanks in advance
Best regards.