iMessage Not Working on MacBook Air & iPad

My iMessage is working perfectly on my iPhone 6, but through my phone number and not my Apple ID. iMessage is not working on my MacBook Air or my iPad. I've tried several times to fix this issue, but I keep getting an "Could not sign in. Please check your network connection and try again" error on my iPad. On my Mac however, nothing is even happening when I signed in to iMessage. The entire login box went gray and is not even progressing. I am concerned as every single device is up to date. I also have no option to connect my iPhone to my iPad, as I know you're supposed to connect them a certain way to get iMessages. I've cleared something in FlashPlayer on my Mac as suggested by a thread, but that didn't do anything. Please help! This has been going on for at least 2 months now.