Check #1 → let your friends check the remote streaming quality preference in their client apps. Many clients default to 720p @ 2 Mbps.
Check #2 → check the Now Playing section of your server dashboard while one of those friends is streaming from your server. Specifically have a look if the stream is displayed as remote or indirect (indirect/relayed connections are limited to 2 Mbps and can be linked to some glitch in a remote access configuration).
https://support.plex.tv/articles/200871837-status-and-dashboard/
If the above indicates there’s a problem with your remote access setup… best start looking into this guide on troubleshooting common remote access issues.
