try installing the RX570 again. Remove it from its slot and install it back in the same first slot and see if the temperature is showing correctly.
Make sure you have the GPU power cable connected from the new PSU.
If it keeps showing "zero" temperature seems to indicate a defective Thermal sensor or circuit on the GPU card.
I would test the GPU card in another computer to see if it is the GPU card or Motherboard that is causing your issue.
Your PSU when it died could have damaged either your Motherboard or GPU card or just before it died completely.
My wife's PSU also died suddenly but only because the PSU Fan stopped working and overheated. No damage to my wife's PC occurred when I installed a new PSU.