What is the upload speed for your Internet service? You should enter that in Settings → Remote Access → Internet upload speed.
Check the settings for remote streaming speed on the Plex client. The default may be limiting playback. On Android mobile it is Settings → Quality → Remote streaming quality. It will be similar on other devices.
Monitor the remote stream via Plex Dashboard → Now Playing + Expanded View.
If the connection is Indirect, then, as @JohnAlex mentions, you are using Plex Relay, which is limited to 2 Mbps.
As far as I can tell (via telnet to the public IP in your logs) you have now successfully forwarded your connection from port 32768 external to 32400 internal (to your Plex server). Does your remote access settings page show that your server is fully accessible from outside your network now, continuously?
You may need to adjust your remote/Internet bandwidth settings in the client’s settings to ensure they are set to a bitrate sufficient to handled the content you are playing.
You may also want to ensure that these settings are set appropriately for your connection in your server’s remote access settings:
