If you are using a device with iOS 9.3.5 or older, you must manually install a compatible LetsEncrypt certificate. Here is a short guide on how to do this:

1. In the LetsEncrypt support forum you will find a thread with the link to the working certificate.
2. The device will show a message: "This website is trying to download a configuration profile. Do you want to allow this?" -> TAP ALLOW
3. The device will show another message: "Review the profile in Settings app if you want to install it." -> TAP CLOSE
4. Open the Settings app. -> NAVIGATE TO PROFILE DOWNLOADED
5. The INSTALL PROFILE screen will be displayed. -> TAP INSTALL in the top right corner. If the device requires a passcode, you must enter it now.
6. A certificate warning will appear -> TAP INSTALL. If a second warning appears -> TAP INSTALL again.
7. The PROFILE INSTALLED screen will appear -> TAP DONE.
8. Before the certificate can be used, the device must trust it. On the device, navigate to Settings > General > About > Certificate Trust Settings.
9. The certificate you just installed will appear on the screen, with a toggle button next to it. Activate the toggle and then TAP CONTINUE.
10. That's it, you now have a working LetsEncrypt certificate installed.