Remote manage a Hyper-V server from a different domain

…don’t!

You can however remove it from its current domain:

netdom remove /d:DomainThatNoLongerExist.COM HYPERVComputerName /FORCE

Then add it in server manager, right click on it and select Manage As… and you can enter the local admin credentials for the Hyper-V box
https://technet.microsoft.com/en-gb/library/hh831453.aspx

Still not entirely sure if you need to run this on the management server…

Set-Item WSMan:\localhost\Client\TrustedHosts -Value -Concatenate
https://technet.microsoft.com/en-GB/library/jj647788.aspx