Looking at the Harbinger LX8 owner's manual it states:
Quote
PLAYBACK & RECORDING VIA USB
The LX8 is a fully ASIO (Windows) and Core Audio (MAC) compliant audio interface and as such do not require any special
driver installation.
. That means Harbinger relies on built-in Windows audio drivers and does not provide equipment specific drivers.

Here are the recording directions:
Quote
Recording:
1. Connect all input signals to the LX Mixer
2. Set the channel level controls and MAIN level fader so that the MAIN level meters show the mix peaking somewhere around 0 dB
3. Make sure the LX Mixer 14 WAVE OUT USB DEVICE is selected as the audio input device within (a) the PC’s system settings and (b) within (Band-in-a-Box and RealBand) the DAW software
4. Arm the stereo DAW track and press record. The LX Mixer’s MAIN MIX will now be recorded by the DAW
For your issue step three is the important one.

Question 1: When you go to the Windows 11 recording audio settings do you see "WAVE OUT USB DEVICE" as a selection? If no then Windows is not seeing the mixer. If Windows doesn't see the mixer then RealBand can't see the mixer. If yes then

Question 2: When you open RealBand and navigate to Options > Preferences > Audio and select the Drivers button do you see "WAVE OUT USB DEVICE" Windows Drivers Audio Input Driver and, is "WAVE OUT USB DEVICE" selected?

Jim Fogle - 2025 BiaB (Build 1128) RB (Build 5) - Ultra+ PAK
DAWs: Cakewalk Sonar - Standalone: Zoom MRS-8
Laptop: i3 Win 10, 8GB ram 500GB HDD
Desktop: i7 Win 11, 12GB ram 256GB SSD, 4 TB HDD
Music at: https://fogle622.wix.com/fogle622-audio-home