The new 300Mbps internet does not work at full speed - cable fault?

Hello. I changed internet from 100Mbps to 300Mbps. I have a TP-Link TL-WDR4300 router and two computers connected by cable to it.
The first computer stands next to the router and is probably connected with a cable added to the router. Internet speed is ok, 280Mbps.
The other computer is in the other room and is connected by a 10 or 15m cable. And here, unfortunately, there is a problem, because on this computer the speed fluctuates around 90Mbps.
Is the cable guilty? If so, someone will recommend which new cable to choose so as not to lose speed. I hope the length doesn't matter here.
I care about the stability and speed of the link.
Greetings.

Does the network card support Gigabit Ethernet? Is the twisted-pair cable at least UTP cat.5e? RJ45 plugs earned in accordance with the ethernet standard?

The network card should support because both computers are the same (the same motherboard and the network card, because integrated). Computers differ only in that on the first (where there is 280Mb / s) is windows 10, and on the second windows 7.
On the computer I found only such a model (?) Of the network card: Intel (R) 82579LM Gigabit Network Connection.
The cable is not what it is, I once bought it on an allegro, it was supposed to work ... and it worked.

Korsarz1975 wrote:

On the computer I found only such a model (?) Of the network card: Intel (R) 82579LM Gigabit Network Connection.

It looks like it supports the Gigabit Ethernet standard. Check if you have forced 100Mbps on the network card.

Korsarz1975 wrote:

The cable is not what it is, I once bought it on an allegro, it was supposed to work ... and it worked.

Upload photos of plugs (colors of wires) and if it has something printed on the outer insulation.

Where to check if 100Mbps is forced? I can't find anywhere.
Photo plug below (sorry for the quality, I do not have a decent camera on hand and the dim light).

Something like this on insulation: communications cable CM 75 ° C 4/24 CAT 5E UTP verified to TIA / EIA 568B 2 & ISO / IEC 11801 052M

On both sides you have the same distribution of wires?

Search for Link speed & duplex - example

I found such a thing. If it changes to 1.0Gbps Full duplex then the internet disconnects. As if I pulled the plug. If I give a car it connects right away, but 100 ...

Replace the cable or earn RJ45 plugs again because you can see that they are not in the best condition.

I forgot to answer your previous question. Yes, the plugs look the same on both sides. I even found an e-mail from Allegro when I bought it, it cost PLN 10 and the auction title was: 10m NETWORK UTP 5e RJ-45 Ethernet CABLE.
When I change to 1.0Gbps Full duplex to Link status: speed not available. When I give the car it combines but 100 ... I have not tested other options but it will be the same because up to 100.
I have checked the driver update yet, as if I have the latest one.
On the other computer, despite the same card and driver, these options look a little different, the speed link is in the same tab as in your photo. There I have a car set up and 300Mbps work.

Maybe some link to a checked cable or a person who earns such? I need 12m the best, 10m will also be stubborn.

Buy the original 10 or 15m patch cord which will have connectors pressed and covered with plastic. Because what you show is someone made himself with a crimp tool.

Example:

Does the category matter? There are cat. 5 and 6 to choose from.
How are they different?

Take UTP 5e - and the differences are described on the Internet I will not write here.