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Tap Install DNS Profile above and tap Allow when Safari asks to download the configuration profile.
Open Settings and tap Profile Downloaded (or go to Settings → General → VPN & Device Management), then tap Install.
Apply the DNS: in Settings → General → VPN & Device Management, open DNS and make sure SideInstaller is selected. Keep it enabled until setup is complete.
Pick any certificate above and tap Install, then confirm the prompt.
Wait for the app to finish installing on your Home Screen.
Go to Settings → General → VPN & Device Management, tap the developer profile, and Trust it.
Launch the app. If one certificate is revoked or won't open, just install with another.
3
Completion
You're all set.
That's it. Open SideInstaller from your Home Screen and use it to install SideStore (or SideStore + LiveContainer), straight from your device. No PC needed.
Once SideStore is installed, the DNS profile is no longer needed. Remove it in Settings → General → VPN & Device Management → SideInstaller → Remove Profile — or keep it and just stop using its DNS by switching DNS back to Automatic in the same menu.