Accidentally clicked 'don't trust' this computer, now the message wont pop up again

It should ask you again if you disconnect it from the computer and then re-connect it. If it doesn't, go to iTunes on your computer, click on Account>View My Account, when the account screen displays, scroll to the bottom of the screen, and click on the Reset button next to the "Reset All Warnings for Buying and Downloading".

Best,

GB

Well, you can try signing out of iTunes and then back in again. You can also try resetting your device:

• On an iPad Pro 11-inch or iPad Pro 12.9-inch: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Power button until the device restarts.
• On an iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo.
• On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for up to 30 seconds, until you see the Apple logo.
• On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for up to 30 seconds, until you see the Apple logo.

If all else fails, contact Apple Support: Contact Apple Support

Best,

GB

Go through the "Get Help" section of: About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support

http://osxdaily.com/2014/03/24/trust-this-computer-alert-ios/ also contains some tips.