Question Slow upload speed through router

[Bence1996]

So my ISP provides me fiber internet. Usually the speedtests are ~900MBit/s download and ~400MBit upload. It works fine if I connect it straight to my PC via cat5E ethernet cable from the modem that the ISP gave me. However if I connect it through my router(modem->router->pc) the upload seems capped at ~100MBit/s, sometimes even lower. The download speed stays the same. On Wi-Fi (5Ghz) I got the correct speeds.
I tried reinstalling the network drivers, the system is basically a clean install.
Yes, I have the so-called problematic 2.5G ethernet adapter on my Mobo.
My router is an Asus RT-AX53U. Tried factory resetting, no QoS enabled. The Firmware is the latest. Every driver is updated on my PC.
My question is: What could I do to dismiss this "upload speed limitation"?
Edit: forgot to mention I use the latest Win11 Home edition.

 

[Zerk2012]

So my ISP provides me fiber internet. Usually the speedtests are ~900MBit/s download and ~400MBit upload. It works fine if I connect it straight to my PC via cat5E ethernet cable from the modem that the ISP gave me. However if I connect it through my router(modem->router->pc) the upload seems capped at ~100MBit/s, sometimes even lower. The download speed stays the same. On Wi-Fi (5Ghz) I got the correct speeds.
I tried reinstalling the network drivers, the system is basically a clean install.
Yes, I have the so-called problematic 2.5G ethernet adapter on my Mobo.
My router is an Asus RT-AX53U. Tried factory resetting, no QoS enabled. The Firmware is the latest. Every driver is updated on my PC.
My question is: What could I do to dismiss this "upload speed limitation"?
Edit: forgot to mention I use the latest Win11 Home edition.

Are you sure your reading it correct?
mBit and mByte 2 different things. Byte = 8 Bits

 

[Bence1996]

Are you sure your reading it correct?
mBit and mByte 2 different things. Byte = 8 Bits

Yes, Im kinda home in IT. However networking is not my speciality. Its a gigabit connection.
If I had 900MB/s download and 400MB/s upload speed i'd be amazed, but its megabit.

 

[USAFRet]

What are you using to test this speed?

Screencaps help.

 

[Bence1996]

What are you using to test this speed?

Screencaps help.

Speedtest.net , tried it woth fast.com too but that wasnt accurate beacuse I live in hungary. Here the most accurate speedometer is speedtest.net

 

[USAFRet]

This sounds like a cable issue.

What specific ethernet cables do you have, and have you tried any others?

 

[Bence1996]

This sounds like a cable issue.

What specific ethernet cables do you have, and have you tried any others?

https://imgur.com/a/GrXT4RE

View: https://imgur.com/a/GrXT4RE

Here are the results. Sorry for the late answers, its midnight here.
I have 2 brand new Cat5E ethernet cables, tried with both.

 

[Bence1996]

https://imgur.com/a/GrXT4RE

View: https://imgur.com/a/GrXT4RE

Here are the results. Sorry for the late answers, its midnight here.
I have 2 brand new Cat5E ethernet cables, tried with both.

https://imgur.com/a/99TVC6v

View: https://imgur.com/a/99TVC6v

UPDATE: I found an another cat5e cable, havent used it because its 5m long, but i tested it out of curiosity, and it really was a cable issue. Kinda funny, the 2 brand new cable I bought some days ago was because the ones I used got rekted in the same time..
Anyway, Thank you guys!

 

[bill001g]

In many ways older cable maybe better ?

There is a massive amount of fake ethernet cable being sold. Copper metal has gotten so expensive and rather than just increase the price of the cables many manufacture find ways to reduce the amount of copper, stuff like thinner wires. A lot of the cable you see being sold as ethernet does not actually meet the required standards for ethernet cables. So unless you spend a lot of time reading the fine print you in effect get ripped off.