$ brew install ngrok/ngrok/ngrok

Sign up (with Github) for a free account: https://ngrok.com/

Then, follow the instructions for step 2. Connect your account on this page: https://dashboard.ngrok.com/get-started/setup

In particular, add your authtoken:

$ ngrok config add-authtoken [LONG PRIVATE-KEY-ISH THING]

I removed the long private-key-looking thing that it gave me in case it is indeed a private key.

Serve!

$ ngrok http 5175
ngrok                                                                                                                    (Ctrl+C to quit)

Check which logged users are accessing your tunnels in real time https://ngrok.com/s/app-users

Session Status                online
Account                       Matt Russell (Plan: Free)
Version                       3.1.0
Region                        United States (us)
Latency                       -
Web Interface                 http://127.0.0.1:4040
Forwarding                    https://2f23-2601-640-8581-24b0-7828-89e1-c9b5-280a.ngrok.io -> http://localhost:5175

Connections                   ttl     opn     rt1     rt5     p50     p90
                              0       0       0.00    0.00    0.00    0.00

Send yourself a text message with the crazy long URL https://2f23-2601-640-8581-24b0-7828-89e1-c9b5-280a.ngrok.io (your URL will be different).

Click on the blue “visit site” button:

And you’re done! Now you can see how absolutely terrible your website looks on a phone before you deploy it.