I'm sorry to hear that you had to go through a factory reset to reconnect your Nest Hub Max. It is very important that we do it especially if there are changes on your Wi-Fi or if you've moved location or in this case, removed from the Google Home app. A few questions: why do you have to remove your Nest Hub Max from the Google Home app? Are there changes done to your Wi-Fi or your account or address?
Doing the factory reset will revert your Nest Hub Max to its default and original settings, please follow the steps below to avoid slight complications when setting it back up.
Keep me posted.