We run the hardware, you run the client. No account, no email, no personal details — a payment and a code, nothing else.
This client does not come from Google Play, and your phone will say so — “Play Protect doesn’t recognise this app’s developer”, or that it could not be scanned. That is about where the file comes from, not what is in it: it is signed, it is the same file every buyer gets, and the same file installs without a word over a cable.
To install it: tap More details, then Install anyway. If your phone asks whether your browser may install apps, say yes.
The app updates itself afterwards, so this is once. Android may ask you a single time to allow settings for an app that did not come from a store — one tap on the app’s own page, and only the first time it offers you an update.