Initiate while on mobile connection or tethered to one (or just leave it connected from home), use while on that WiFi.

EDIT: I figured this out because I brought my laptop from home to do a few things while at work that needed it. I noticed that my Tailscale connection (initially established at home) was working just fine. That's when I realized that it was the initial authentication that was blocked, not the service.

My phone is usually on my tailnet and my iPad is always on it (and using my home exit node), as a result. Using the exit node has a modest but noticeable effect on battery life, but just being connected is maybe 2% of battery a day. Negligible.