From the install doc:

My way around this is to use ngrok.com, which is a quick and simple way to create a temporary VPN with a domain and SSL. On the free plan your domain changes often, which will break federation every time you reconnect.

You can now get 1 free static url with ngrok’s free plan. The ‘free’ part of it is that it needs relaunching after a certain period of time (every 2 days, maybe?) but it works well enough to develop things.