OctoApp is a superb native mobile remote for a printer you already host yourself, beautiful UX, cheap, open-source, privacy-friendly and EU-made. If you have one or two printers and just want a great phone app, it's a lovely choice, and it even pairs fine alongside SimplyPrint.
SimplyPrint is a full cloud platform, not just an app, with native iOS and Android apps of its own plus a print queue, AutoPrint continuous printing, a full cloud slicer, AI failure detection, filament inventory with NFC, a maintenance system, multi-user roles with SSO and a school plan. The moment you're past one printer, one person, or want any of that management layer, you've outgrown what an app can do.