The timeout is probably coming from the mod_reqtimeout Apache module, which is used to prevent DoS attacks like SlowLoris:
The cPanel team recommends temporarily commenting out the module, and restarting the Apache server:
====
# sed -i '/^L/{s/^/#/}' /etc/apache2/conf.modules.d/375_mod_reqtimeout.conf
# /scripts/restartsrv_httpd
====
You'll either need to customize mod_reqtimeout, or remove it.
The package name is 'ea-apache24-mod_reqtimeout'.
See:
https://documentation.cpanel.net/display/EA4/How+To+Mitigate+Slowloris+Attacks