iPad External Display feature not working (More similar questions!)

Not all third-party USB-C hubs work well with iPad. Unless your hub specifications explicitly state compatibility with iPad and iPadOS, it may not be suitable.

If you are able, you would be well advised to test the monitor with an Apple USB-C Digital AV Multiport Adapter:

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

If the problem clears when using the same monitor and the Apple Adapter, you will have demonstrated that the issue lies with the hub.

I have experienced something similar myself on just one occasion. For me, the resolution was to perform multiple forced-restarts of the iPad - both with the monitor connected and disconnected from the iPad.

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.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

Otherwise, I have no additional suggestions at this time.