Cannot use Silent Remote Sending

I’m trying to create a form that loops, using “Silent Remote Sending Settings”. I will post what I am doing and hopefully somebody can point out to me what I am doing wrong, or even suggest what I might try.

I can get it to work when I use “Enable posted data to be sent as a query string on the redirect URL”, but not when I use “Posted form data can be sent silently to a remote form using the method GET or POST”.

This is Form 2 of 2. The URL is http://stmartindeporres.niftywebdesigns.com/religious-education/religious-education-online-registration/additional-child-registration

Step 1: I enable “Redirect Settings” to redirect the form to itself at http://stmartindeporres.niftywebdesigns.com/religious-education/religious-education-online-registration/additional-child-registration

Step 2: I have set “Silent Remote Sending” to Enabled: Method POST.

Step 3: Silent Remote URL is set to the same as the redirct URL http://stmartindeporres.niftywebdesigns.com/religious-education/religious-education-online-registration/additional-child-registration

Step 4: “Silent send fields to rename” is… Silent send fields to rename
envelope-number=2envelope-number (‘2’ is the number of the form I’m trying to send it to.)

I’ve tried several variations of this setup. Any help on this is appreciated.

https://wordpress.org/plugins/si-contact-form/