If you have an NFL GAME PASS Standard subscription then you can watch games, shows and more on 2 different devices at the same time, by using the same IP. However, you can upgrade via My Account to the Ultimate Tier to enable 5 concurrent streams across 2 IP addresses.
If you are having issues watching on two devices, please check that they are connected to the same internet source – for example, if you are watching NFL content on your TV that is running from your home Wi-Fi and you want to watch on your mobile phone as well, your phone needs to be running from your home Wi-Fi, not your mobile data.
You can see all the GAME PASS content here.