DP 2.1 not reaching full 80 GBs bandwidth

[CasualGunGuy]

Hello, I’m not sure if this is a niche problem so I’ll try my best to articulate a properly. 

I have an Asus PG27UCDM latest firmware update (June 2025). Along with an Asus RTX astral 5090. 
both support display port 2.1 full bandwidth 80 GBs 

however, I am only able to reach 4K at 240 Hz without HDR enabled (NO DSC). 
it will not allow me to enable HDR until I turn on DSC. 
but with display port 2.1 80 GBs capabilities, I shouldn’t have to worry about DSC. 

after installing GPUZ, it is showing that I have four lanes at 10 GB each with my RTX 5090 however it should be 20 GB each equaling to 80 GB

im using the DP cable that came with the monitor. Everything has the latest drivers and bios updates. 
I tried unplugging all my additional monitors. 
each lane is stuck at 10gbs 

any solutions or answers that I missed? 

[Zenpharaoh]

I’m having the same issue. Any updates?

[CasualGunGuy]

I was using the wrong cable. It comes with two display port cables. Only one of them is rated for the full 80GBs bandwidth for DP2.1

its should have a tag in it listed DP80 on the cable. Unfortunately it’s only 1 meter long. But that solved the issue. You’re probably using a DP2.1 cable but a lower specification that isn’t the full 80GB