I have turned off my MBX servers for DAG failover DR testing so my mailbox db is dismounted but I cannot log into ECP although my CAS serve

This is expected behavior. In 2013 world, CAS can do nothing without mailbox server.

• Marked as answer by KingMT 19 hours 50 minutes ago

I thought as much, shame as would be nice to still manage organization from the ECP with the databases dismounted as we then need to revert to the shell...

Personally I'm 90% on shell while both are available. Too many things not avaible on ECP.