Are you trying to login to the Boost device itself from the admin credentials on the tab on the rear of the device? These should be different from My Telus credentials.
I am alos having the exacatly same problems with mywifi app. I can login to mytelus on my desktop but can't login to mywifi app on my iphone6. The app says : Unable to login to this aoo with your myaccount settings. Please try another account. I don't have any answers from telus too. They have the worst customer service. Why do I have to wait over an hour to talk to a human .
I don't recall a login to My Telus (or My Account), just the login to the Boost Device, using the credentials under the tab on the back.
Edit: updated to most recent version of the App and had no trouble to login.