"will try putting a pullup resistor" sounds like more trial and error experimentation. Seems like it would be a whole lot more productive to get the data sheet for the 433MHz device and determine exactly what the specified output drive capabilities and characteristics are and then devise a connection to the MCU that is compatible to that specification.

If you purchased some type of hobby device that may not include data sheet of schematic then you need to look at the physical design of the device in detail. If it is some type of single IC chip design then look up the data sheet for that chip to learn about its drive capabilities. If the output pin of the unit is implemented in some type of transistor and passives circuit then you may have to trace the circuit from the PC board connections.

The chances of finding the correct hookup through trial and error are smaller than you may think. Engineering work done using this methodology is a hack at best and and very often fails in four corner testing of voltage and temperature extremes.

- Michael Karas