How far do you get with the import process? Do you get the Sign In screen after selecting your broker? Since you've had issues with two different brokers, I wonder if you're not getting a good connection to the brokers' servers. Here's How to Turn Off Anti-Virus Programs, which may resolve your issue.
The setting to allow 'third party access' would be in your broker account settings. Be sure to double-click on the right account in the search results.
If you're using TurboTax Online, clear your Cache and Cookies between attempts.
@Dashir