Microsoft Exchange 2010 Migration to Exchange 2019 Standard

Attempting a two-step migration from Exchange 2010 (on Windows 2008 R2 Server) Server to Exchange 2016 (2016 Windows Server) then to Exchange 2019 Standard. Everything will remain on-premise. All pre-requisite config has been done (Windows Features, .NetFramework, Unified Communications Managed API), I've prepared AD and Installed Exchange 2016 on Server 2016 using GUI method. I've set autodiscover for CAS, configured VirD Urls and have access is EAC and OWA. DNS and Firewall changes have been made to accommodate new Exchange 2016. Problems: [login to view URL] Mailboxes are failing and won't connect to Exchange 2. Users are getting security/autodiscover warnings [login to view URL] default mailboxes on 2010 DBs will connect to excahnge but shared mailboxes have either disappeared or can't be accessed through outlook 2016 and outlook 2019. [login to view URL] Mailboxes on 2016 DB will only authenticate through Microsoft account therefore won't connect to On-Promise Exchange 2016 through Outlook 2016 and Outlook 2019 but I am able to access mail using Exchange 2016 OWA. Notes: 1. Send/Recieve Connectors on exchange 2016 not configured correctly? 2. Outlook Anywhere Auth set to Basic and allows SSL Offloading. 3. Autodiscover URi is different for both exchange servers and I'm able to reach both 4. Internal URL is the same for both Exchange servers 5. External URL is null for Exchange 2010 Server 6. 10 Accepted domains and 1 External Relay (forwarded to accepted domain through provider) Task at hand: 1. Troubleshoot Exchange2016 to Client Outlook Connectivity 2. Set-up successful co-existence 3. Re-establish shared mailboxes and proper connections to accepted domains 4. Migrate/Upgrade to Exchange 2019 Server Standard and be independent of Exchange 2010 and Exchange 2016 which will be decommissioned.