Getting a "certificate is not trusted" error for iOS Distribution Certificate

To support an older version of an iOS app, I needed to install Mojave with Xcode 9.

I create a CSR and from there generate an iOS Distribution certificate online, which I download and double click to install in Keychain. However, when viewing in Keychain, it shows in red "iPhone Distribution: CompanyName certificate is not trusted".

I'm not sure what I'm doing wrong as I follow the instructions to the T and have done this in the past with success.

NOTE: I tried this with Sierra as well and got the same issue. It only seems to work on Catalina, which doesn't help me because I need an older version of Xcode.
Thanks Patel
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U save my life, PATEL.
THANK YOU PATEL!!!
God bless you and your family @patelgaurav4u_ios
Yes. Many thanks. Days of head scratching. Apple, why so hard?
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Patel, You the man!!
@patelgaurav4u_ios Thanks a lot, you're a life saver
Thank you patelgaurav4u_ios!

Patel, you're the best of the best!
Thank you patelgaurav4u_ios .
10x, patelgaurav4u_ios. 
Thanks patelgaurav4u_ios. Thanks for not very much, Apple.

Patel, you are a legend mate. From NZ with many thanks ~

Getting a "certificate is not trusted" error for iOS Distribution Certificate
 
 
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