I have downloaded Simplify3D (Simplify3D-4.0.0-linux-x64-installer.run) and installed it using the terminal. I've reached the directory and then installed it with sudo prefix:
cd /home/user/Downloads chmod +x Simplify3D-4.0.0-linux-x64-installer.run sudo ./Simplify3D-4.0.0-linux-x64-installer.run
The program launched, accepted the linked account's password and everything worked perfectly.
Firstly, I don't understand why I cannot find Simplify3D via search on the computer. It is neither shown in the Ubuntu Software list. The launcher is on the Desktop, and if I delete it from there I cannot use the software anymore. Every time I launch the program prompts a "first-time setup window" (where I choose the 3D-printer model), for some reason the computer doesn't remember my setup.
OS is Ubuntu 16.04 LTS. It has no other big problem at all.
The real problem seems to be the changing IP-address: after restarting the computer, launching Simplify3D prompts "Account Limit Reached" window (this software has a license for two computers) and says
Please uninstall the software from one of your previous installations before proceeding. Visiting the Simplify3D website and signing in shows the account is linked to this Linux computer (and another account linked to my co-worker's Windows).
Anyways, the changing IP-address is not an Ubuntu OS related question. I will troubleshoot that with the ISP (internet service provider), try to set a static lease IP-address and of course contact Simplify3D support.