Get url param no longer works when using add_rewrite_rule

Question

I don’t know why my question was not well formatted and can not modify it.
Here’s the more readable one:

I’m using WP Admin DIVI builder to create a page (named get-region-page), in which Pod shortcut is used to retrieve the url param. For example:

[pods name="region" slug="{@get.regionparam}" field="region_name"]

which gives apparently the same result as using these in the php template of the page:

$_GET['regionparam']

With a redirection 301 to that page url: /get-region-page&regionparam=$region_slug, everything works fine, I can retrieve the url parameter.
The problem arrives when I replace the 301 redirection with a rewrite rule for that page in functions.php (with 1234 the page id for get-region-page):

add_rewrite_rule('({some-custom-regex})/?$','index.php?page_id=1234&regionparam=$matches[1]','top');

…the parameter regionparam becomes null. Only this works:

get_query_var('regionparam');

but no longer this

$_GET['regionparam'];

So I wonder if there is any way I can retrieve that get_query_var(‘regionparam’) for my first pod shortcut to rework again in DIVI builder?
Thank you for your help!