Matrix Synapse NGINX Troubleshoot

My Matrix homeserver is almost ready: it runs on a DietPi box with Synapse installed, the database is populated, federation is enabled, and DNS/DDNS records are in place. The one piece still blocking public federation is the NGINX reverse-proxy layer... (Budget: $30 - $250 USD, Jobs: Apache, DNS, JSON, Linux, Nginx, PHP, SSL, System Administration)