Hi @barmstrong
Low water level in a toilet bowl is indicative of a partially blocked sewer pipe or air vent.
If the other toilet is filling instead of draining this also indicates a blockage in the pipes somewhere.
You could try clearing it using a [https://www.curranplumbing.com.au/how-to-use-a-plunger-to-clear-a-blocked-toilet/|plunger].
If still no good then you may have to try using some other [https://www.jetsetplumbing.com.au/blog/read-this-before-you-use-a-drain-snake-or-electric-eel/|options]¹ or calling a plumber as they should have the necessary equipment to clear drains.
¹Option shown only to show example of what you can use - not an endorsement of the service.