I can’t extend my iPad Pro 2018 to my monitor, only mirrors

By model number you appear to have an iPad Pro 11" - of which all models should support Stage Manager.

Your initial comment as to connecting your iPad to the monitor "by HDMI via a cheap USB-C hub" is perhaps a concern - as its functionality and capability are an unknown. The hub and/or cables making the connection may themselves be the source of your difficulties.

As a diagnostic exercise, substitution of both your hub with a known good HDMI Adapter or USB-C hub and good quality HDMI 2.1 cable would be advisable. By example, if connecting via HDMI:

• Apple USB-C Digital AV Multiport Adapter

https://store.apple.com/xc/product/MUF82ZM/A

Otherwise, with your existing arrangement, try a forced-restart of your iPad - as this will often clear an isolated iPadOS software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

If the problem persists after performing the restart, it may be necessary to reset your iPad settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPad settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPad to factory settings:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Should the issue remain unresolved, you might be well advised to seek assistance directly from Apple Support - but be aware that you will likely be advised to substitute the USB-C hub and cables as previously suggested.

You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians may be able to assist:

Genius Bar Reservation and Apple Support Options - Apple

Due to limitations of internal hardware, the new Stage Manager feature (that provides the full extended desktop) introduced with iPadOS 16.1 is only available for these models of iPad:

• iPad Air (5th generation and later)
• iPad Pro 11-inch (all generations)
• iPad Pro 12.9-inch (3rd generation and later)

Feature availability is usually limited by system hardware - CPU and available RAM being critical to some advanced functionality.

Other iPad models only provide screen mirroring.

Not sure which iPad you have? Identify your iPad model.

Additional information can be found here:

Turn Stage Manager on or off on your iPad - Apple Support

For iPad models that support Stage Manager, your monitor’s screen resolution will impact the available settings and external screen capabilities.  Ensure that your iPad has been updated to iPadOS 17.4.1 or later - as some earlier versions of iPadOS have known issues.

Additionally, while undocumented, testing has demonstrated that 4:3 monitors with a display resolution of 1280x1024 and below are currently only usable with iPad screen mirroring; as such, associated settings to disable screen mirroring in favour of Stage Manager are not available in iPad Display settings.

When used with a 16:9 widescreen monitor, with a native display resolution of 1920x1080 or greater, the full set of Display settings - and access to Stage Manager - should become available.

The iPad User Guide lists the upper resolution limit as 6K - but the Guide offers no lower limit, or suggestion of aspect-ratio, being a factor for compatibility:

Connect iPad to a display with a cable - Apple Support

This discussion thread may be of additional interest:

iPad Extended Desktop not working - Apple Community

In conclusion, your monitor must be capable of 16:9 widescreen 1920x1080 (FHD) to support iPadOS16 Stage Manager extended desktop. Unless the monitor meets the minimum required specifications, you will not see the full range of controls in iPad settings.  

When mirroring your iPad display to an external monitor, expect to see black bars to infill where the iPad screen doesn’t match the aspect ratio of monitor screen.

Please confirm the iPad model number - and installed version of iPadOS:

Settings > General > About >

• Model Number
• iPadOS Version