no Windows PC
Do you have a Mac? Garmin Express works on Windows and MacOS.
How did you set up you F35 initially? Clearly if you had access to Garmin Connect you then had access to a computer? AFAIK you need access to a computer or a phone to initially set up your device and register it to your Garmin Connect account.
While the process has undoubtedly changed over the years, there has always been a need to do this at the very least with a computer - if memory serves me right.
You should have an Add Device option at the bottom so tap that and use it to add your new deviceand I can't see this option in my Garmin Connect account.