To close the loop with my issue here, it appears that the Asus Radeon RX-550
doesn't have any PWM control. The fan speed is constant and can't be changed in
Windows using the GPU Tweak II software either. A curious bit is that while the
socket for the fan connector has 4 pins, only 2 are actually driving the fan
(however, I have owned graphics cards before that were able to adjust the fan
speed by adjusting the voltage using just the two +/- cables).

After discussing with Asus Support and the shop where I purchased it from, we
came to the conclusion that all 550 series cards in the market don't have PWM
control whereas higher models do. I replaced it with a (Gigabyte) RX-560 which
is very silent. `sensors` now shows the RPM for the card. My understanding is
that all models starting with 560 onwards support fan control, regardless of
the manufacturer.