Yes, I have assemble the cooler myself.
My Room temperature is around 30°c as l am leaving in Bangladesh.
I'm from India. Room temperature without AC is about the same as yours.
I have the 3600 and when I run Blender(very CPU heavy) or Cinebench, with the side panel open, mine reaches 89-92C and doesn't go any higher and I haven't applied my own thermal paste to it. There has to be something wrong.
For a 3900X that temperature is higher than the maximum temperature of 95C for Ryzen processors, and the CPU should throttle to stop the temperature from rising.
So, I would suggest you re-seat the cooler again, and use some force to make the contact very tight this time. You could also change the thermal paste but the default thermal compound isn't responsible for this high-temperature reading, so applying new thermal paste won't help you that much.
Then check if you have mistakenly changed the fan speed in BIOS settings, or any motherboard software.