Working from Turkey this summer and they are known to block random hosts/IP addresses. I can get to github.com without problem but something about both getting flyctl and doing the signup are blocked from Turkey. Not a concern for me because #icanuseavpn (grin) but flagging for others.
[robs@fedora ~]$ curl -L https://fly.io/install.sh | sh
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 1475 0 1475 0 0 1520 0 --:–:-- --:–:-- --:–:-- 1520
curl: (28) Failed to connect to github.com port 443 after 132220 ms: Couldn’t connect to server
[robs@fedora ~]$ fly auth signup
Error: Post “https://api.fly.io/api/v1/cli_sessions”: dial tcp 77.83.143.220:443: i/o timeout
[robs@fedora ~]$ nordvpn c us # enables VPN, everything works
Connecting to United States #9808 (us9808.nordvpn.com)
You are connected to United States #9808 (us9808.nordvpn.com)!
[robs@fedora ~]$ fly auth signup
Opening https://fly.io/app/auth/cli/151dcaf3e7cd33cd9345857d53cc8d3f …