Can you check on the client machines firewall to make sure that ‘Remote Desktop- user mode (TCP-In)’ and ‘Remote Desktop- user mode (UDP-In) are allowed in the inbound rules.

https://www.experts-exchange.com/questions/28706199/Remote-desktop-does-not-work-with-windows-10-upgraded-computers.html

Remote Desktop can’t find the computer “FQDN”.