One of the new additions to RX 8 is Horizontal Scrolling, which allows you to use your mouse or trackpad/trackball to move across a zoomed-in timeline.
If this isn't working for you, it's likely you have a mouse that doesn't have specific support for this movement. RX 8 uses the functionality pulled straight from your Operating System, so if you're unable to scroll sideways in other applications (like your DAW or another Host Application), it won't work in RX either.
If your mouse has proprietary software associated with it (eg Razer Synapse, Logitech Options), you can configure Horizontal Scrolling there!
Current workarounds for horizontal movement are seen below:
The Grab and Drag tool
This RX 8 Help Documentation page shows more about this tool.
The top navigation bar (Waveform Overview)
This RX 7 Help Documentation page shows more about this tool.