You deployed an LDAP server on Compute Engine that is reachable via TLS through port...

Add the network tag allow-udp-636 to the VM instance running the LDAP server.

Create a route called allow-udp-636 and set the next hop to be the VM instance running the LDAP server.

Add a network tag of your choice to the instance. Create a firewall rule to allow ingress on UDP port 636 for that network tag.

Add a network tag of your choice to the instance running the LDAP server. Create a firewall rule to allow egress on UDP port 636 for that network tag.