How do I install an older version of an app on an iPhone?
It sounds like the Apple Support article explains what you need to do to get the app in the version you want. 


Go to the App Store on your iPhone. 
Press Updates and then press Purchased.
When you get there, it should show your Apple account and it will say
My Purchases. 
Press that and it will show you all of your apps
Find Trello and try to download it. If a version of the app that is compatible with your version of iOS is available on the App Store, it will let you download it to your phone.
If the version is not available, go to the iTunes App Store, find the app and click Ratings and Reviews. 
There is a button there that says App Support. Click that and then put in a query to the developer. They may be able to let you directly download the version of the app that you need from them.


If all of this fails, possibly other options like Todoist have the older version of their app that you can access.

This is unfortunately not going to help you with this app, as it does not sound like you had downloaded this app prior to this question, but it will help you in the event that you accidentally upgrade an app that will then no longer work with your version of iOS.


Go to iTunes and Delete the new version of the App from your library and also from your drive.
Go to the Music>iTunes>iTunes Media>Mobile Applications folder on your drive.
In that folder will be all of the current apps plus a folder called Previous Mobile Applications.
Previous Mobile Applications stores all of the old versions of your Apps each time you do an update through iTunes.
Find the version of the App that works with your version of iOS and drag and drop it onto iTunes. iTunes will reload that version of the App into your library and move the file back into the Mobile Applications Directory.
Delete the app from your phone, then just sync your phone and the old app should be loaded in. You may need to click on the phone and apps in iTunes and have iTunes add it.


Best of luck on getting the app.