Launch Disk Utility and see if you can manually mount the TM drive. Sometimes external drives won't mount automatically.
Another possibility is that the file system is damaged. Unfortunately macOS & Disk Utility don't provide very good error messages in this regard....many times it may even silently fail when you try to mount it. After attempting to manually mount the TM volume, then try to run First Aid on it since the TM volume will most likely be unlocked.
With what version of macOS was this TM backup drive originally created? And what file system is it using.....APFS or MacOS Extended (Journaled)?
Encrypted or not encrypted? If encrypted and you save the TM backup password in the KeyChain, then you may want to delete the TM backup password entry from the KeyChain....or possibly the new Passwords app (Apple is still transitioning from KeyChain app to Passwords app).
FYI, Disk Utility unfortunately hides the physical drives by default with recent versions of macOS. Within Disk Utility click "View" and select "Show All Devices" so that the physical drives appear on the left pane of Disk Utility. If the physical drive is not seen, then you have a hardware issue of some sort.
Definitely try connecting the TM drive directly to the computer. You may also want to disconnect all other external devices in case one of them is causing a problem.
I've also never heard of that hub brand before. I personally prefer to stick with the well known brands that have a good reputation. Macs can be very particular about third party devices. Or one of the items is bad (hub, cable, drive, another connected device).
