Connectivity issues with new Exchange 2019 running in 2016 environment

Installed exchange 2019 into existing exchange 2016 environment.  Now having some issues.  email connectivity goes up and down randomly.  Which causes issues with active sync on phones, owa and outlook.  It is also kicking back emails saying the user cannot be found (on new users created)(unless you use owa, then it sends fine).  Offline Address book not showing any new users.  I have only moved a few mailboxes to the new exchange 2019, the rest are still on 2016.  I noticed however, that outlook shows that it is using the new server automatically, and was not sure if that was part of the issue.