Is the WeMos D1 using an ESP8266 chip? I think they all run on 3.3V. Maybe test the pin with your multimeter.
If your relay is just a solenoid connecting switch you'll need a diode to prevent the current from flowing in the wrong direction.
Let us know when you're not new here anymore so we can make fun of you.
If you are using a relay module though, all of that circuitry may already exist. ..... I think you might be trying to interface directly to the relay.
I think it depends. Normally a relay takes a lot more current than a digital pin can put out. You would need a transistor to interface to it. If you are using a relay module though, all of that circuitry may already exist. In that case, it would be a simple interface issue. In my opinion, I think you might be trying to interface directly to the relay. In that case, you won't have enough current to energize the coil and turn on the relay.
but 3V should actually be high enough to be a "high" on the 5V side.
So maybe the real question is in fact (although you say it's working on an Arduino?...) are you actually putting a LOW on the IN pin to close it?
You can also try physically taking the signal input line and directly connecting it to 3.3V then removing it putting it in GND to test it.
It's possible there might have been something wrong with the 5V supply you were using.