On the corsair units I have ran of the i series max pump speeds range from 2750-3000rpm. H100I, H110I.
H150I is 2760ish rpm.
Some units can control each fan independently and on some control both fans but only 1 rpm reading.
The 3 pin plug with the single wire is only a tach/sense wire that feeds the pump speed to the cpu_fan header (per instructs) or to another header which I found out.
On my Asus R S X470-F Gaming I hooked up my h110i per the instructions to boot it up to a cpu fan error.
I set that header to ignore and was JUMPED ON here on Th for doing so.
I now just plug my rear exhaust fan to it and set a custom curve in bios for approx 950ish rpm across all temps, but always monitor cpu temp with Icue, Hwinfo64, Or RM.
1 thing people do not realize, is the temp reading in Icue / CLink4 is the coolant temp not the Cpu temp by Default (which in his case would be the H115 coolant).
It can be set to the cpu temp but my fans reacted to the ryzen temp spikes.
So maybe check that setting.
It makes for a little fun trying to set a custom curve of fans speeds when the temps are based off the coolant temp vs Cpu temp to have the fans react the way they need to.
You can also choose fixed% or fixed rpm.
Check those settings to see if you can or can not adjust the fans.
Using the presets I always set the pump speed to max which can be labeled differently between which cooler / software it is.
So Performance/ now Extreme in Icue.
Fans Balanced / Performance gives a good range to cover what is being done at the time, Browsing, Gaming, Cpu intensive work but are all still based off of the coolant temp.
If he has followed the instructions and has it set right he should have control over his fans and have fan speeds, pump speed, Temp info.
If he doesn't have any readings then
The next thing I would check is the usb connections.
Next device manager in the usb section at the bottom look for a corsair entry.
At 1 time I don't remember if h100iv2 or the h110i, but pretty sure it was CLINK4 after an update needed that device to be re-enabled then worked correctly again.
Reinstalling Icue / Clink4 also has helped.
You can do a repair or uninstall from the Corsair installer you use to install the software.
If it asks if you want to remove any profiles or config files let it, then reboot.
I run ccleaner after rebooting then install Corsair again.
If all has been setup right and still don't have the control then call Corsair Tech Support let them know the issue and what you have done to resolve it and depending on it's age it can be turned to a RMA very easily.
From dropping a RMA cooler off at the Ups store to recieving the replacement on 1 was 13 days which I know dosen't help when there is no stock cooler for the 5900x as backup.
Good luck will be watching to see the outcome.