The PageSpeed nodule for Nginx on the host server can be installed by manually installing the module and then rebuilding Nginx with it.
Official guide: https://www.modpagespeed.com/doc/build_ngx_pagespeed_from_source
If you installed ModSecurity, it's important to rebuild Nginx with the same flags as it was originally built with when setting ModSecurity.
After installing the PageSpeed module, simply add to the main nginx.conf file:
load_module "modules/ngx_pagespeed.so";