I was able to register with (American) Amex. I just tried to change the credit card registered with my SmartEx account and I was NOT able to register with (Canadian) TD First Class VISA nor (American) Chase Sapphire Preferred VISA using the SmartEx app. I can try to do the same with my desktop browser etc.

SmartEX uses 3D Secure technology to validate the credit cards and It's also possible that the banks are automatically blocking foreign transactions they think are fraudulent so might need to give the banks a call to see if they see anything on their side.