It is important to mention that accessing and downloading this cert from any other browser other than Safari will result in an error. Using Safari when downloading and installing the cert will create installation prompts for the certificate.

To install the Securly SSL certificate:

Navigate to securly.com/ssl and download the certificate by clicking "Download the certificate".
Open your Settings app and navigate to General > VPN & Device Management
Tap the securly.com profile that you downloaded in step 1.
Next tap the Install button in the top right hand corner.
Tap Install once again in the pop-up.
You Securly SSL certificate should now be fully installed.
Next, you would need to trust the certificate by navigating to Settings > General > About > Certificate Trust Settings and toggling the switch to enable the securly.com certificate.
The Securly SSL certificate will now be fully trusted. You can view the certificate under General > VPN & Device Management as a Configuration Profile.

How to deploy Securly SSL certificate to iOS