After upgrading a RDP Broker server ( and webgateway )  from 2012 R2 – 2019 and then the session host ,  trying to connect to the Collection with the file from the web interface I was greeted by the error

“the remote resource can’t be reached”

The collection only contained one session host , testing out the connection to that session host individually worked

I then tested the RDP file downloaded from webaccess again and it worked which means the sessionhost seemed to need more time to register or reboot the servers again!