I’m in the UK, using a Mifi router from Three, 2.4 and 5Ghz. No doubt the exact same thing, but in the UK and it works fine.
By the way, your phone and app have zero to do with the functionality of your system. Yes you set it up with it, and no doubt you can cause it to go wrong during set-up, but for a working system, you can have no app at all, and turn your phone completely off, and the system will keep working just as it did, so an app update should not be able to break the system.
If an app update did something to stop the app working properly on your phone, check to see if the ‘system’ is still working, bar the phone. Walk in front of the cameras. Does the IR light come on. Does the record light come on, if you have it enabled. Do you see traffic through your router, when monitoring your Blink devices’ stats?
Do you have another phone? Maybe an Android version. If not, borrow a friend or family members, any sort of phone, that’s high enough in terms of its OS version. Install Blink. Log in with your credentials. Does it work? If it does, you know support were talking nonsense and your ‘system’ is working fine, and it’s just an app issue. Log out, uninstall, and return phone. You can do this anywhere, so just visit a friend to test.
No friends, do you have a PC? If you do, install MEmu and then the Blink app. MEmu is a great Android emulator, very fast, and will allow you to see if it’s your app or system that died.
Once you have reached that point, you can concentrate on the right angle of attack. If it works on an Android emulator, continue setup with the 4th camera, as you planned, and then concentrate on solving the phone specific app issue.