one dc goes down and things stop working

We have 4 dcs an mixture of windows 2008 and 2016 

When one dc goes down things stop working

Any non cache login no longer work

Any thin clients can not login states can not find domain 

once it back on everything works again

I test this by disabling the virtual nic and re enable it

ALl are dcs in vm environment of esxi 6.7

This DC was once a rdoc but promoto long time ago

when I run 

repadmin /showrepl   it all successful

when I run repadmin /syncall /AdeP

How can we fix this we can not have a single point  of failure

Also any other dc  I test by disabling nic everything still works its just this one dc 

Please help