XB8 getting 1G on ports 1-3 but only 350Mbps on port 4

@flatlander3​ windows 10, Linux Mint, and ASUS RT-AX86U that was pulling 1.5G from Shentel 

@ChristianB81​ Stuff to poke at maybe.  Might have hangups with full duplex and Xfinity gear.  They don't actually have any hardware documentation for their outsourced rental stuff.  You could try messing around with duplex settings, instead of leaving it to auto, or set full duplex manually and see if anything changes.  If it does, then see if you can match that on your router's config.  

Also depending on your NIC hardware, TCP/LSO offloading might be a problem too.  Just enable checksum offloading and see if that helps.  Run direct to your XB8 when you are testing this.  If it's a laptop you are using, plug it in.  Clocks/bus speed changes on battery on a lot of gear.     

Tried locking it into 1G Full Duplex, offload was already enabled Tx and Rx

@ChristianB81​ How about manually clocking it for 1Gbps.  Can you at least get that out of the port?   If not:

Xfinity uses a lot of refurbs for rental gear because the default plan is to swap out the modem for their database problems and other issues.  Their hardware is also known for Ethernet problems.  You may have ended up with two other people's problems.  No company on earth could afford to scrap hardware at the rate they do, so they're passing em out.    

The quick way is to swap it out at an Xfinity store.  Maybe you'll get one that wasn't floating around on a truck. 

I can try that I guess, the ones they hand me are wrapped in plastic, no box 

Also saw there was an "Auto" setting for WAN as apparently if you have fiber they use 4 to bring it in and use 4 as the WAN

I changed it from Auto to DOCSIS and it didn't make a difference

I have since added a Fedora Computer and a Chromebook to the mix to no avail

Looks like this is a semi common problem that usually ends up with people abandoning it