Trying to set up Exchange 2013 server to migrate from Exchange 2010

I am attempting to configure a new Exchange 13 server to migrate from 2010. Here is what is done so far (Exchange 10 is the live server, 13 is the new one)

Send/receive connectors between the 2 servers are functional. I can send internal emails between the 2 and outside emails can route through the 2010 box to test mailboxes on the 2013 box. Same for the other way, I can send out of the 2013 box.

Webmail for the test users in the 2013 box is functional.

AutodiscoverURI is set up on the 2013 box to be correct

My problem I am encountering is I cannot get the Outlook 2013 clients to connect to the mailboxes on the 2013 server. If I try to add a mailbox or simply log in directly with those users, it says "Exchange must be online" then "Name cannot be resolved"

I have checked both the internal and External DNS for "exchange13" (the name of the 2013 box) and they are of the correct IP. I can go on the computer I am testing the mailboxes on and do and nslookup that points it to the correct IP. I have tried on multiple computers and gotten the same issue.

If I attempt to do the mailbox wizard setup, it does prompt me to approve the exchange13 certificate, but will just continuously prompt for credentials, which I enter, then the box simply re-appears.

I am at a complete loss of what to do right now. I've tried so many "fixes" or suggestions I saw from other forums, nothing works.

Can you log in to the mailboxes through OWA? Here’s an article for you to check whether the migration have been finished Migration From Exchange 2010 to Exchange 2016 (& 2013) If the migration has been finished, the issue is caused because the Autodiscover data is not updated automatically with the new location of the mailbox. CAS 2013 servers are still proxying the request to the 2010 mailbox servers. You can recycle MSExchangeAutodiscoverAppPool on the CAS 2013 servers to resolve it. Open IIS Manager, click application pool and find MSExchangeAutodiscoverAppPool. Click recycle. For details, https://somoit.net/exchange/exchange-2013-mailbox-migration-outlook-cannot-connect