What OS are you installing to?  I am installing on my private server in a VMware VM on esxi behind a natted cisco firewall.  The first scenario was on CENTOS 7 with the chat on the same system unit,  Virtualmin and Webmin were in place too.   I gave up on trying to work it into the UNA interface.  That said,  Rocket chat did work,  but I dont want to take the time to instruct you on how to fall short.    Let me know what you are working with and maybe I can help.   in the meantime,  this attempt that I am making now is on Debian 10 buster..  

HI LeonidS, UNA is on it's own server,  I created a Debian 10 Buster VM as a host for rocket chat, Set it up and installed Rocket Chat.  Rocket Chat is working with Ningx. I set up SSL and Reverse Proxy consistent with most of the instructions I have come across, Rocket chat is working in SSL and also works using http on port 3000  It is visibly appearing in UNA now and the button for una login is present.   THE PROBLEM:  When I click on the button whether or not from inside of an UNA block or from full screen outside of una, it looks like Rocket chat is logging in as una user but after several seconds,  it just returns to the login for rocket chat in both inside una and outside scenarios.   There is an error message in the browser debugger:  Blocked autofocusing on a form control in a cross-origin subframe.     And in my rocketchat access log: some Failed - connection refused errors popped up when I was trying to use https in root url in rocketchat.service without the port, following daemon, nginx, rocketchat reloads and   I ended up seeing nginx error about bad gateway.  I am chasing my tail a bit.