I have an internal LAMP (Ubuntu 18.04) server that I use for various personal projects. It has always been exposed directly on ports 80 and 443. It hosts 4 sites (Apache virtual hosts) and I use CloudFlare full SSL for the domains. I issued the Let's Encrypt certs using certbot. This was all done following tutorials online as I've never been a sysadmin.
Last night a friend put an NGINX server up and all traffic on ports 80 and 443 go to it instead. We're working on some projects together and now have several servers in my network, hence the nginx reverse proxy. I have been asked to simply use ssl passthrough to re-enable access to my sites.
I know where the config files are located and I know how to restart the nginx service, but that's about it. I have never worked with NGINX and I have no idea what configuration to use or how to proceed.