xb8 multi-gig port 4 only gives 300mps where ports 1-3 with same cables/connections deliver close to 1000mbps

X2 multi-gig level service, a slightly beat up xb8 brought today by xfinity

using cat8 patch cable directly from port 4 of the xb8 to a laptop with a 2.5G nic adapter, only achieve 300mbps

   -xb8 admin tool indicates the lan port is achieving a 2500mbps connection, but observed speed never exceeds 375mbps

using same patch cable directly from port 1,2,3 of the xb8 to same laptop observed speed > 900mbps

  - same observed speed issue whether bridge mode engaged or disengaged

using cat8 patch cable direct from port 4 to the multi-gig port WAN on a netgear RAXE500 router, and then connecting patch cable from a multi-gig LAN port on the router to the laptop with the 2.5G nic, still only 300mpbs.  This is despite the xb8 LAN status showing a 2500mbps connection established on port 4, and the router wan status showing a 2500mbps connection established with the xb8

using same connection config except connecting from the 1.0g ports 1,2,3 on the XB8 results in  > 900 mbps observed speed

I can conceive a few scenarios, but does anyone have other ideas?

1)  My xb8 is somehow mechanically faulty on port 4?

2) Xfinity is somehow not provisioning my specific xb8 to allow multi-gig use on port 4, despite my subscribing to its "X2" multi-gig service tier?

3) Xfinity has not provisioned the physical network in my local geography to allow port 4 to work on customers with xb8s or xb7s?

Since 900mbps speed is available using this XB8's 1.0G ports directly to a laptop in gateway mode and through a 3-rd party router in bridge mode, its not the cables, its not the router

The fact that port 4 can only deliver 300mbps through the same cables to the same devices and routers suggests...what?

Thanks in advance