Can I back up my phone without being connected to WiFi?

...without being connected to WiFi

It is possible for iPhone 12 or later to use 5G phone data, not your iPhone X.

Apple's support document Backup iPhone says

On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular

For older iPhones with 4G or earlier the data transfer is too slow and the connections sub-optimal for the important work of backing up your iPhone.

Otherwise, you can backup your iPhone to a computer with iTunes or for more recent Macs via the Finder.

You can back it up to your computer.

You can not back up to iCloud using cellular data unless you have an iPhone 12 or newer with a 5G data plan.