If you use Nginx proxy manager this is what fixed my federation issues:

location / {
      # The default ports:
      # lemmy_ui_port: 1235(not used here)
      # lemmy_port: 8536
	  # The cooler port:
	  # lemmy_ui_port: 1236

      set $proxpass "http://172.16.7.20:1236";
      if ($http_accept ~ "^application/.*$") {
        set $proxpass "http://172.16.7.20:8536";
      }
      if ($request_method = POST) {
        set $proxpass "http://172.16.7.20:8536";
      }
      proxy_pass $proxpass;

      rewrite ^(.+)/+$ $1 permanent;

      # Send actual client IP upstream
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header Host $host;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }