Where are you getting your unifi package from? It’s not in the official Ubuntu repositories. From what I can tell, UniFi does not have an official repository for Ubuntu 18.04; it has a repository for Debian 9, and recommends using this script for Ubuntu. Before you risk the security of your server, make sure that your repository is for Ubuntu 18.04 Bionic Beaver and not any other Linux distribution. That means the word bionic should be in the line you add to your sources.list to make the repository work.

If that isn’t helpful, then in order to downgrade with APT, look at all available versions with apt-cache showpkg <package>, then install the correct one with sudo apt install <package>=<version>. A fair warning: this will mean that the package you downgrade won’t ever be upgraded, which could leave your server insecure!