Arduino vs Wemos Fan control
The IRF540 is not suitable to be controlled by a 3.3V device because it has a gate-source-threshold voltage of 2-4V (that's the voltage at which the mosfet starts to conduct, but you need to be above that voltage). While this worked at 5V logic level on the arduino, the 3.3V are not enough to make the mosfet switch entirely.

A suitable mosfet has a threshold voltage below 1V. Also for a fan, you eon't need a big mosfet rated for 30A, a tiny one will do it.