Sorry for the delayed response.
The issue reported by TPGNBN should not be related to your case. Their setup involved VLAN configuration on the WAN interface for ISP authentication, whereas your issue is that the link is negotiating at 100 Mbps.
To help narrow this down, could you please confirm the following:
- Have you tested with different Ethernet cables? If possible, try the cable included with the Flint 2 or a shorter cable that is known to work properly with other devices.
- If the issue persists, please try placing an unmanaged (dumb) switch between the Flint 2 and the ISP modem to see whether this changes the link speed.
- Please try configuring LAN 2 (the 1 Gbps port) as the WAN interface by following this tutorial, then connect it to your ISP modem and check whether the issue still occurs.
GL-MT6000 (Flint2) How change LAN2 - LAN5 port to WAN - #28 by bruce - When using the same Ethernet cable and modem port, do other devices negotiate at 1 Gbps normally?