I managed to install Ubuntu Touch on a Fairphone 2 using the following method. I'm not sure every step is necessary, but I'll list them just in case.

• Laptop running Ubuntu 18.04
• Installed ubports-installer_0.1.9-beta_amd64.deb via command line (not Ubuntu Software) using 'sudo dpkg -i ubports-installer_0.1.9-beta_amd64.deb'. 1.20-beta failed repeatedly despite trying the deb, snap, and various permutations of this list.
• I had installed and reinstalled various versions of the OS on the Fairphone 2, but a fresh install of Fairphone Android manual switcher (fp2-sibon-18.04.1-manual-switcher) worked.
• Enabled developer mode with USB debugging enabled.
• Selected 'trust device every time'.
• Allowed ADB access to apps.
• Launched Ubports using CLI: 'sudo ubports-installer'
• Stepped through the wizard, using 15.04-stable, and wipe device.

Now I'm enjoying Ubuntu Touch - thank you!