As the D3400 doesn't have WiFi, remote control over Snapbridge isn't possible.
Have you read the directions for using the ML-L3 on page 95 onwards in the manual? I am not sure if B works with the remote, you might have to use T to control the shutter opening and closing from the remote.
No, I had not. Just did, and see that one needs to set the proper release mode. Good news in that the shutter opened when I set to 2 second delayed response, but only after pressing the ML-L3 button several times. I'm not seeing anything happen when set to quick response mode. I recall seeing instructions on a web page about pressing the ML-L3 3 times to "arm" it. I'm going to get a new battery before doing any more tests. Can you point me to a guide for the ML-L3, if this is not it? Thanks
Using an Optional ML-L3 Remote Control