WAN Speed stuck at 100M Full Duplex on RT-AC68U

[ptmy2]

I am running Asuswrt-Merlin 380.57_0 on an Asus RT-AC68U router. Everything's working fine, except when I looked under Tools->sysinfo, I noticed my WAN Link State is at 100M Full Duplex. The LAN speeds are at 1000M Full Duplex. Why is the WAN speed not also at 1000M Full Duplex? Is there any way to fix this?

 

[RMerlin]

What's your modem?

 

[ptmy2]

It's an Arris TM822G/CT connected to Comcast Xfinity cable.

 

[RMerlin]

That modem does have a gigabit interface. Try swapping the Ethernet cable between the modem and the router, make sure the cable is Cat 5e or higher.

Otherwise, could be that the modem was configured with a 100 Mbps interface instead of being left on auto-negotiate.

 

[ptmy2]

It's the wire. Thanks for your help.