PeerTube have updated their official app, you can now sign in with a PeerTube account
Unfortunately Apple & Google are being crap:
Apple App Store users can only sign in on the built-in server list, Apple won't allow manually adding servers
Google Play users can manually add unlisted servers
F-Droid users get a *much* wider server list and can add servers manually
You can avoid app store problems by using PeerTube's web app: https://fedi.tips/how-to-install-web-apps-for-mastodon-and-the-fediverse