Please share your config.g file.
Check your config.g file by runnig M98 P"config.g" to see if you get any error messages.
One cause of all thermistors reading -273 is that there is a short between VSSA and a higher voltage, for example a heater wire. Such shorts are quite common in 3D printers, which is why Duets are protected against them. So disconnect all thermistors from the Duet, then test using resistors again.
If you still always get a reading of -273, check the resistance between VSSA and GND. It should be only a few ohms, it's the resistance of the VSSA polyfuse..