Here’s how mine is connected:
With an actual picture:
That said, since I have a newer Chamberlain opener, I had to solder the cable that you see coming out from the box on the right to the button on my opener for this to work.
For the box, I used the “box” from an old AC/DC converter plug that was no longer working. I removed the guts from it and replaced them with the Uni.
This is basically, what I did first... I don't get it...
So it sounds like you did everything right… From the app, you can switch the relay to closed and open with no issues? Have you tried a resistance meter on the black cables to make sure that it properly made the required contact?
Oh, and you are using Direct Current, not Alternating Current, right? (I think also that the polarity is important - as long as the red light turns on and it is accessible in the app, it should be okay).
So, rewired everything. The shelly is accessible in the app. So far so good. When I measured after turning on the shelly, recognized a voltage drop of 1V. As mentioned before, on my door buzzer I could measure 14 V, my shelly is connected to 12 V. Could this be the issue? And if so, would a stronger power supply solve this issue?
Ok, got through several threads in the shelly forum. So, I'm an idiot. Why? Let me walk you through it:
I used the wrong power supply. I was sure to use an AC supply, but I used DC. In and of itself, not a big deal, if there wasn't point No. 2.
If the Shelly is connected to DC it can hardly switch AC. I still don't get why, but this seemed to be the problem.
So, what's next? I'm getting myself an AC power supply and see if this changes anything. I'll hope for the best and keep you guys posted.
So if I understand, you need it to feed 14V AC to your buzzer?
If so, then the 12V DC should remain as is on the Red/Black cables.
I would think that one end of the 14V AC should be connected to one end of the relay and the other to the return of the buzzer. Does that make sense?
What I have is an opener, so it only needs to connect the two cables together temporarily to open the door.
It does, totally. But what seems to be the problem, is that the shelly uni doesn't like to be connected to DC, while trying to switch AC. At least that's what I figured out while going through several threads at the shelly community.
Same here, that's what drove me crazy...
Let's see if the AC-Power supply that I've ordered will make any difference. I'll keep you posted.
thats your issue
It cant / shouldn't have AC connected to its outputs
saying that it then says this!
Ok, the AC power supply wasn't the problem either... And I just don't get it. What bugs me even more, is the fact, that as soon as I wire the shelly to my door buzzer, it's not buzzing anymore... When it's removed, everything works fine again. And again, pure confusion...
So thinking about doorbells, they require an inline resistor in order to work. Without seeing your setup I figured I'd throw that out there.
Possible... I honestly have no idea... 
I'm going to make a picture tomorrow and post it here. Let's see if anyone has a clue. So far, I'm out of ideas how to control it with the shelly uni and thinking about building a little robot, that pushes the button for me. Like the switchbot.
Neither of those will work from what I can see. You can't mix the inputs and outputs like that. In fact, your existing button probably should not even be touched in this setup. You are misunderstanding how to wire this.
In my opinion, this Shelly device isn't really suited for this task. It is overly complicated, and doesn't appear to have just a set of relay contacts which are what is needed here. There are easier to wire devices like this device that don't require a bunch of farting around for the same price. Amazon.com
That being said, do you have a digital multimeter, and do you know how to use one? If so, I can probably guide you through this.
The problem here is that the UNI does not have dry contacts (mechanical relays) and what they use to switch is 100mA Max, most of the time a "buzz in door system" uses a mechanical electro magnet and many older ones are directly connected to the button in each apartment, these things will pull a lot more than the 100mA the UNI can deliver and is most likely the problem here. If you want to use the UNI, you should get a low power relay to wire in between the UNI and the door "buzzer". Just make sure the relay does not need more than 100mA to engage and be sure to get a mechanical one.
Yes, I do... 
Absolutely agree. Until now, every time I tried to get something done with the Shelly Uni, it just got complicated and never brought me the result I've wanted... 
Yeah, I got one and know how to use it. If you could help me, that would be awesome. 
I didn't see that this was an apartment door buzzer until I re-read it and with what @nclark said above.
You will need to do what he said and get a relay that isolates this device from your door system, and to carry the load of this door device. This shelly is not suitable by itself for this task, it will likely burn up (if it hasn't already) by trying to use this as a standalone device.
But looking at the MHCOZY, it should do the trick. It has dry contacts, and when I look at the advertisement, door security systems, closing systems etc. are listed. So it should work, right?
Assuming you have a button on your door you can wire in any dry contact. I use this one for my gate opener.
Pricy right now
I've tried the MHCOZY today. It was a direct success. Just plug and play. 
Edit:
For readers later on; In my case, the regular door buzzer stopped working, as soon as I connected the MHCOZY, for what ever reason. To keep the regular door buzzer-function, a used the manual switch, which is connected to the relay. I put everything into a box and squished a plastic button over the switch to access it.
The MHCOZY was set to switch mode. Within Hubitat, I quickly wrote a rule, turning the switch off, three seconds, after it was turned on. This keeps the regular buzzer function, with more than enough time to enter the building.
Using the Hubitat Dashboard from @jpage4500, I quickly created a widget on my smartphone, for a quick access (don't forget to enable cloud mode). Now I don't need a key anymore, to get into my building.
Download the Hubitat app
