Just got my Cosmo yesterday, super excited to upgrade from my old, failing original Pixel.
Had a few things set up in my Cosmo last night, then finally shut it down and installed the SIM from my Pixel. When it booted up, it told me I needed to install the FI app, so I agreed and went through the activation process. Seemed like it got most of the way done, but got to a point where there was an error ID:080. Called tech support, got same story, the Cosmo is just not supported. Retried the Fi registration several times, but it never succeeded.
After that I did a factory reset, rebooted, but this time did not agree to install the FI app. Shortly after I got a Cosmo notification that I needed to configure the SIM card. I only have the one, so SIM 1 for voice, SIM 1 for data. It seemed to be hit or miss after that, sent a couple texts, received a couple texts (although one had some weird gibberish at the end). Made a couple test calls, but 2/4 inbound calls went straight to voicemail (one of them may have been when I was rebooting, but one was definitely when I was sitting there waiting for the incoming call).
After the above test where I had used the Cosmo settings to configure the SIM card, I installed the Fi app again, and this time it registered with no an errors. Now the Fi app is installed, I got the two voicemails form the missed calls. I still have some testing to do, have to make sure both inbound and outbound texts and calls will still work, but I'm hopefull.
Also, if the pre-installed "Messaging" app (green icon) doesn't work with Fi, I will try installing the "Messages" app from Google LLC (blue icon) that I've been using on my Pixel. Seems like it may have better compatibility with Fi. @Grench did you happen to try this method for sending texts with the Fi app installed?