Problems with Jetpack and HTTP/1.1

My web host recently switched off access to HTTP/1.0 since many spam bots tend to use it. Ever since, I’d been having some issues with two wordpress plugins, Jetpack and Simple Lightbox.

After talking with my web host I was able to fix my issue with SLB simply by modifying two files to force HTTP/1.1 instead of 1.0:

./wp-includes/class-IXR.php
./wp-includes/class-snoopy.php

Unfortunately I’m still having no luck getting Jetpack to connect. When I try, I get this error:

Your website needs to be publicly accessible to use Jetpack: site_inaccessible

Error Details: The Jetpack server was unable to communicate with your site [HTTP 400]. Ask your web host if they allow connections from WordPress.com. If you need further assistance, contact Jetpack Support: https://jetpack.me/support/

My web host does not block access, and xmlrpc returns the correct line when I hit it with my browser:
https://wing-on-wing.com/xmlrpc.php > “XML-RPC server accepts POST requests only.”

In ./wp-includes/class-wp-xmlrpc-server.php the only time it will return a 400 is when “Insufficient arguments passed to this XML-RPC method.”

I’m really stuck here and would like to be able to use Jetpack again!
Thanks for any help.