Never heard of an infotainer temp gauge but I have a dash mounted outside temp device and it's hard wired to a sensor mounted behind the front 'bumper' if you can call it a bumper. Anyway, it's solidly made so if I had --- in the display, of course I'd check the users guide to find out what that means, first. If no help, and based on my knowledge of electronics I would think that was an 'Over range' error message. BTW, typically the number of dashes indicate the range, so if it only shows -- and it's over 100F where you are, it won't display. But I'd start looking for damage to the outside sensor, and I'd check any connectors between the gauge and the sensor.
Likely you just have dirty or corroded connection somewhere.