Strange Issue: Getting full 300mbps for my Ignite Internet plan on Wi-Fi, but not on Ethernet connection

[RainbowTrout]

A bit of history: Before a recent complimentary upgrade to 300Mbps down/100Mbps up, I was getting the full 150Mbps down/15Mbps up on my Ignite Internet via Ethernet connection (through a switch, see below) to my desktop computer.

My setup consists of the XB6 Gateway (Technicolor version using Broadcom chipset), with a TP-Link TL-SG105 switch that was provided by Rogers to connect more ethernet devices to the gateway (since the gateway is limited to 2 LAN ports), and a desktop computer connected with an Ethernet cable via its 1Gpbs capable LAN port which uses the Intel I211-AT controller chip.

As already mentioned, before the upgrade to 300Mbps download, I was getting the full 150Mbps speed to the desktop via Ethernet connection through the switch whenever I did a speedtest.

Ever since the speed upgrade to 300Mbps, now I'm getting 50Mbps down/100Mbps up through the switch. If I disconnect the switch (with all the other devices that are connected to it) and plug the Ethernet cable from the desktop directly into the XB6 gateway, I'm still only getting 100Mbps down/100Mbps up and not 300Mbps down.

But if I do a speedtest via Wi-Fi (to my Wi-Fi 6 capable laptop for example), I'm getting the full 300Mbps.

If I log into the XB6 Gateway and go to Gateway > Hardware > LAN it shows that the connection speed on LAN Ethernet Port 1 that the switch is connected to, is 1000Mbps.

I did call Rogers technical support and went through a bunch of troubleshooting steps, but this hasn't been resolved satisfactorily yet. So I thought I'd try asking here for any advice, before contacting Rogers again.

I've also tried changing the Windows Auto-Tuning setting on the desktop and rebooting, as suggested in other discussions, but that didn't have any effect.

My best guess is that somehow the Auto-Negotiation between the gateway and the switch, or the gateway and the desktop if it's connected directly, isn't working correctly now for some reason? When it was fine before. Again, I was getting 150Mbps download to the desktop through the switch, before the speed upgrade to 300Mbps.

But, frankly, I'm at a bit of a loss here as to what the issue is, exactly.

***Edited Labels***

[RainbowTrout · Accepted solution]

EDIT: Never mind, I fixed it! Replaced the Ethernet cable from the desktop to the switch and now it's working properly, getting full speed (even through the switch) as expected.

I find this pretty odd, though. In the sense that I've never experienced an Ethernet cable "degrade" or go bad before. It was working perfectly fine at 150Mbps not too long ago. No amount of fiddling with the cable helped, it needed to be replaced. Inspected it and there didn't seem to be any damage, no oxidation on the contacts as far as I could tell. So not sure how this "degradation" could have happened.