As the Synology UI currently does not allow the addition of multiple DNS servers for active directory connectivity we usually customize the file /etc/resolv.conf
Please login to your synology as root via SSH and then change the resolv.conf file to the following:
nameserver 10.198.71.100
nameserver 10.198.72.100
nameserver 10.198.73.100
search domain.comdivision.com
options timeout:1
Please note that 10.198.7x.100 are the three DNS servers and the timeout has been reduced to 1 second... You should also setup the search domain.