Ethernet Link Speed Capped at 100 Mbps Intel good read

https://www.intel.com/content/www/us/en/support/articles/000058908/ethernet-products/intel-killer-ethernet-products.html

If this is set correctly and your link speed still reads as 100 Mbps, then the issue is almost always an issue with an Ethernet cable, though it could be a few things.

Before troubleshooting any further, check the connection of the Ethernet cable into every piece of networking equipment, as well as your computer. Make sure each cable is labeled Cat 6 or Cat 7. Unplug each Ethernet cable and plug it back in. Make sure that your cable is pressed firmly into the Ethernet jack. You should hear and feel a very audible click sound when the cable is in place. If you do not feel and hear this, then this cable should be considered broken, and you need to replace it.

Even if you believe that your Ethernet cabling is perfectly fine, and even if this cabling worked fine before, swapping it out for another, proven cable, or a brand new Cat 6 cable, will almost certainly solve the issue with the minimum of troubleshooting and headache. This is a very, very common occurrence when troubleshooting Gigabit Ethernet, and it nearly always comes down to one cable being the culprit. Note that this includes all cabling between the machine and the router, including any cabling in before and after any switches, or on the other side of any wall jacks, and behind the wall. However, if you were getting Gigabit using a particular setup and suddenly, with no changes whatsoever, your link speed is now 100 Mbps, then it’s probably only one cable that is currently having issues, and it is likely one that is exposed.

The fastest way to rule out any problems with anything other than cabling is to connect your machine directly into your modem with a single, proven Cat 6 or better Ethernet cable