The UDID is your iPhone’s ID; the certificate is issued for it. A small profile reads it. Usually one of three things gets in the way: the site is not open in Safari, a VPN is on, or the profile downloaded but was not installed.
What to do
Open the site in Safari. The profile will not download from Telegram, Instagram or Chrome. Copy the link and paste it into Safari.
Turn off VPN. Just for the minute while the UDID is being read.
Allow the download. Safari asks whether to allow it — tap Allow.
Install the profile in Settings. Settings → “Profile Downloaded” at the top → Install → passcode. Safari returns you to the site with your UDID.
Still not working?
No “Profile Downloaded” line? Check Settings → General → VPN & Device Management. If the iPhone asks you to wait an hour, that is the iOS security delay.