Hi
Please try performing a clean reinstallation of firmware version 4.8.3 using U-Boot, and configure only the basic settings for initial testing.
- U-Boot guide: What should I do if my router is bricked - GL.iNet Router Docs 4
- Firmware download: GL.iNet download center
If the issue continues, try setting LAN1 or LAN2 as the WAN port to see if it improves performance.
For reference: How to change LAN2/LAN5 port to WAN