That is the wrong place in the router. Back out anything you changed in that section and restart the router.
AT&T BGW320 Config and Plex Media Server Settings
See this post: Changed ISP - No Remote Access with AT&T BGW 320-500 - #3 by FordGuy61
Synology Config
Note: Not the same system as in the post above, so it has a different IP address (.49 vs .55). Adjust accordingly.
Synology Control Panel → Network
General Tab
Network Interface Tab
Edit Network Interface
