I use this IOT relay. No firmware changes are required. The relay is hooked up to a set of fan pins (there are at least three different sets on the Rambo board), and the relay is controlled by issuing M106/M107 g-code commands. The fan pins by default are 12V pins, so to make the kind of relay you reference work, you will need to do one of two things: 1) purchase a 12V version of that relay, or 2) make a simple modification to the firmware so that one set of the fan pins gets reassigned to 5V pins.
In the Marlin firmware in pins_Rambo.h, this is the section that would get edited:
#ifndef FAN_PIN #define FAN_PIN 8 #endif #define FAN1_PIN 6 #define FAN2_PIN 2
Here is a picture with some labeled 5V pins:
44, 45, and 46 are a good choice since they are PWM pins.