Well, the firmware and carrier update are for ios 12.x. I am not sure why you had to manually change it. If you would like to make it work with Koodo, I would suggest you bring the phone to the latest ios (ios14) or 13.x when Apple still allows.

Apple no longer provides software updates for iPhone 6 and 6 Plus. Support ended at iOS 12.x.

There is a repository of iPhone firmware files available. An earlier baseband version might solve the problem. 

Firmware/iPhone/12.x

Well, Apple is still providing  latest software update ios14 for the 6s and 6s plus https://9to5mac.com/guides/ios-14/

The problem with software  for “non-jailbroken” Apple iPhones is they can only upgrade to the version that Apple allows. As of now, Apple only allows signing in for iOS 14 or 13.7 (for 6s plus) https://www.imore.com/apple-no-longer-signing-ios-1361-so-its-ios-137-or-nothing-you  If someone is staying at 12.xx with iphone 6s plus, he/she cannot downgrade/upgrade to any version except the one allowed by Apple.

If someone wants to downgrade or use other version of iOS of their choice, they have to jailbreak and the process isn’t enjoyable.